Fix bug in String.Equals unrolling for certain single-char strings

[EgorBo]

Fixes#78169 bug

For single-char strings with "signed" bit (e.g. ((ushort)'鉄') >> 15) we used to do sign-extension instead of zero-extension 😞
From my understanding, only single-char strings (length == 1) of certain two-bytes languages/symbols are affected (looks like mostly around CJK languages). Still, definitely needs to be backported to 7.0

[ghost]

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Issue Details

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Fixes #78169 bug

For single-char strings with "signed" bit (e.g. ((ushort)'鉄') >> 15) we used to do sign-extension instead of zero-extension 😞
From my understanding, only single-char strings (length == 1) of certain languages are affected. Still, definitely needs to be backported.

Author:	EgorBo
Assignees:	-
Labels:	area-CodeGen-coreclr
Milestone:	-

[EgorBo]

/azp run runtime-coreclr outerloop, runtime-coreclr jitstress, runtime-coreclr gcstress0x3-gcstress0xc

[azure-pipelines]

Azure Pipelines successfully started running 3 pipeline(s).

[danmoseley]

Coding guidelines I think say to always use ASCII, ie use \uNNNN to avoid accidental garbling later. I don't know whether we are actually consistent. But in this case it would perhaps be clearer.

[EgorBo]

Thanks, changed to \u9244

[am11]

There are some existing non-ASCII chars in this file:
https://gist.github.com/am11/dddd538ad08af8c914ba1144adf74263#file-stringequals-cs

[EgorBo]

Thanks! Let's keep this PR smaller for easier/cleaner backport. @am11 feel free to file a PR to clean that up after

[am11]

feel free to file a PR to clean that up after

#78265

[EgorBo]

@dotnet/jit-contrib @jakobbotsch PTAL, I'm going to backport it once merged

[EgorBo]

/backport to release/7.0

[github-actions]

Started backporting to release/7.0: https://github.com/dotnet/runtime/actions/runs/3446166446

[github-actions]

@EgorBo backporting to release/7.0 failed, the patch most likely resulted in conflicts:

$ git am --3way --ignore-whitespace --keep-non-patch changes.patch
Applying: Use cast nodes
Using index info to reconstruct a base tree...
A	src/coreclr/jit/importervectorization.cpp
Falling back to patching base and 3-way merge...
Auto-merging src/coreclr/jit/importer_vectorization.cpp
CONFLICT (content): Merge conflict in src/coreclr/jit/importer_vectorization.cpp
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
Patch failed at 0001 Use cast nodes
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".
Error: The process '/usr/bin/git' failed with exit code 128

Please backport manually!

[github-actions]

@EgorBo an error occurred while backporting to release/7.0, please check the run log for details!

Error: git am failed, most likely due to a merge conflict.

[EgorBo]

SPMI failures are due to today's JIT-EE guid update