ARM64/ARM64EC vectorized `minmax_element()` family mishandles unsigned elements

[StephanTLavavej]

In the not-for-review #6371, I encountered a randomized test failure in ARM64EC vectorized min_element(). I hardcoded this seed on top of main and reproduced this failure deterministically in the not-for-review #6372. Then I got the precise input that reproduces this failure, and verified that it fails for plain ARM64 too:

std::min_element() failed for vector<unsigned char> input:
155, 153, 248, 150, 189, 140, 247, 178, 244, 164, 226, 214, 239, 215, 176, 127

Click to expand original log:

name: std.tests/VSO_0000000_vector_algorithms
output: Build setup steps:
Build steps:
Command: "C:\Program Files\Microsoft Visual Studio\18\Insiders\VC\Tools\MSVC\14.52.36530\bin\HostARM64\ARM64\cl.EXE" "C:\a\_work\1\s\tests\std\tests\VSO_0000000_vector_algorithms\test.cpp" "-IE:\stlBuild\out\inc" "-IC:\a\_work\1\s\llvm-project\libcxx\test\support" "-IC:\a\_work\1\s\tests\std\include" "/nologo" "/Od" "/W4" "/w14061" "/w14242" "/w14265" "/w14582" "/w14583" "/w14587" "/w14588" "/w14749" "/w14841" "/w14842" "/w15038" "/w15214" "/w15215" "/w15216" "/w15217" "/w15262" "/sdl" "/WX" "/D_ENABLE_STL_INTERNAL_CHECK" "/bigobj" "/FIforce_include.hpp" "/w14365" "/w14668" "/w15267" "/D_ENFORCE_FACET_SPECIALIZATIONS=1" "/EHsc" "/MDd" "/D_ITERATOR_DEBUG_LEVEL=2" "/std:c++20" "/permissive-" "/w14640" "/Zc:threadSafeInit-" "/arm64EC" "/wd28301" "-FeE:\stlBuild\tests\std\tests\VSO_0000000_vector_algorithms\Output\054\VSO_0000000_vector_algorithms.exe" "-link" "-LIBPATH:E:\stlBuild\out\lib\arm64ec" "-LIBPATH:C:\Program Files\Microsoft Visual Studio\18\Insiders\VC\Tools\MSVC\14.52.36530\lib\arm64" "/MANIFEST:EMBED" "/OPT:REF,NOICF" "/machine:arm64ec"
Exit Code: 0 (0x0)
Standard Output:
--
test.cpp
--

Intellisense response file steps:
Test setup steps:
Test steps:
Test step failed unexpectedly.
Command: "E:\stlBuild\tests\std\tests\VSO_0000000_vector_algorithms\Output\054\VSO_0000000_vector_algorithms.exe"
Exit Code: 3 (0x3)
Standard Output:
--
This is a randomized test.
DO NOT IGNORE/RERUN ANY FAILURES.
You must report them to the STL maintainers.

Seed vector: 3356369755,3848971058,1259264218,2402281943,3053744863,1434038012,1370400208,942575957,1162353574,2229769496,2914173799,1559606643,2181804207,141501947,362047169,3202026107,135169558,1923025753,2657321462,529087294,1948307924,1646602262,1256472567,3928544273,808722250,153798203,3086794346,3551505781,3937367038,4248187415,1832215151,911851231,2712842899,2842349105,1062391024,2873581491,3066738274,2991669960,3114333273,1587900168,1272089752,411424740,92810746,3867161257,1659049079,960853619,3026519663,3827007609,3718533934,1445912890,70617560,379462789,2546239587,2082446646,1947649241,3962379562,463764735,2953308329,1055612931,2907965664,844871964,3717101277,3093970926,3898354009,1558082261,3131388974,2658399524,2099000452,1542236179,55900861,2858587320,891703103,2962117772,80107642,204416291,794586963,605083623,807237438,2290023941,2253562912,4202905403,1767359583,3861728986,1490343746,1037127564,1342130107,917587045,2123642814,732727921,1666696914,3512198135,3088071480,12535932,1355722938,1614954222,2816496323,1509261921,3095483927,4025258325,3737754445,3552783602,708549790,1367623855,1850288331,3209478397,429864036,2889880328,2519052311,1937036081,3820528648,4094538420,3087427709,2429115697,4285227434,4045490667,1035752091,2587375556,3620701450,3601667906,2124515128,610284770,2111878862,2101950416,1073201438,914456354,3850466530,2300168682,2125836691,439423149,1005240327,2863985002,1741843053,1974435050,2037694121,607455708,2418510279,3547401786,506333459,220487911,2964227520,3778831470,4018649171,4116487176,1278429181,3617142449,2898791326,495442403,1868165690,1162683806,2331721049,576884234,395506800,1044606013,3970134337,2044067483,607966642,2783059238,2609850724,3303046362,1132529737,1559784530,1825239780,2284346580,1458632731,1763931480,4060951630,2826275853,634331608,1444406349,893150402,2751652575,2345554950,2260030336,560102889,1998229620,820022135,4213805721,2283042431,52686058,3427838800,3882981202,2139056759,104953453,1852273611,2580233003,1307590897,674813102,1336310300,3282289672,681617294,2996556094,3418896200,3923309687,4292700019,563509089,706166904,4139972301,2460527069,217057057,2898004837,2299881379,3778410384,3318187754,143681192,2996761381,2540904366,1293920608,1201285332,1490723386,3691161293,2738019671,2268779047,1775018477,1878391245,2362534121,3737730944,3171948230,711401952,3122073782,1105897327,4222941132,3357595933,3033816911,784967652,553042647,3525850925,1204197706,1709411258,1831376986,1025133928,1095042275,2065227917,2965894916,672023683,467454179,3539266189,317452084,3088769003,2855133613,2736957358,734446004,2476300736,4072974775,1391327770,3210628290,4022971092,1528459190,1311300225,3196246445,2067269631,3534164852,2549792670,497061368,139004917,1177268294,3388218231,1596912311,3762516210,1703801558,2696493320,1026357734,2541060009,2847978756,1913813582,3034729324,1353437905,2296195417,776900446,457394557,1683903247,3632294448,3165117828,2021469897,1574939510,2518540484,3726484384,3327802471,3246281419,3182634912,3960501144,3772861322,2601490194,2415977375,2245355564,2765915561,2884449533,2665012041,3932208016,3354079637,3295088945,2739856147,567286549,1144812561,3666618847,692304243,3119402505,745309817,3159135416,2704378452,1126493547,3405142975,2767607822,3081749123,4137288733,1140102660,3571062893,190570016,2089343369,3195552555,2213389894,3362820739,955151449,2474309873,3641989752,2053636773,2696357607,4151368762,2044296957,614494746,3230827872,1984843437,660783258,1666491793,3330484273,2609451166,1687123480,539761524,1846248369,2189300796,3617547411,4236193427,607307416,379545226,3739900040,2891179612,1201152717,751628666,2444274515,1505717683,716859871,3187819282,362229246,2376762324,3265395497,3865421298,1718933283,757405823,3868866125,3927619024,3717105204,4049730279,3772286259,3888400991,1480129015,4074920807,2385602439,360018923,769865002,3603426055,1683881311,293887490,94607185,1684563041,4228043625,1547624110,1067987678,3909692167,3831076524,2375797938,3027616964,2973545811,1529221750,181117984,1046812220,763548270,1317384548,1254318272,1034739732,3472275787,2894051461,3196371142,2187627793,20640647,2625934759,1974688238,3390060743,640393585,2096375305,210609970,2088757633,4101425581,1909459835,1480988936,2922461172,3120736628,3743094457,4021799272,1700205119,59175518,1740664023,3565992953,3318784316,2824602357,2467851514,90544403,1696979373,784588894,1768610360,3384644717,2795670093,2964178894,234099163,3116922059,1674389962,266005074,2337720195,818405464,3076354369,1596195364,1662868987,1644848768,3706976223,2070523517,477360744,646490681,395676292,2649814891,2503770250,581533669,3537710675,868771147,2591463605,1366809229,4270921544,1471749337,1303961927,252810515,400599123,1305901041,338644684,520183201,2106276135,2646864142,1106668979,21744225,1825268152,3274756622,4183226506,1092947171,3762422968,3008443919,1724755533,2714409633,879342143,4261818063,2477555343,3066983876,209896238,3456661954,3274329276,2724323716,4035563084,3029478406,4022620770,17709068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--
Standard Error:
--
Assertion failed: expected_min == actual_min, file C:\a\_work\1\s\tests\std\include\test_min_max_element_support.hpp, line 92
abort() has been called--