Enable constant folding in Tier0

[EgorBo]

From what I see so far that we should mainly focus on two things in Tier0 (except for debug-friendly code):

1. Emit less basic-block/class probes counters due to cache contention in multi-thread env
2. Remove dead branches early because most of the time in Tier0 we spend inside VM calls (resolve tokens, type loadings, etc) so the more code we remove the less we touch VM. For instance, here are first seconds of Paint.NET's launch forced to stay in tier0:

(purple for methods with CEEInfo:: - VM stuff)

Example of a code this PR improves in Tier0:

staticvoidMain(){if(typeof(int)==typeof(float))Console.WriteLine();}

was:

; Assembly listing for method P:Main()G_M1402_IG01: 55pushrbp 4883EC20 subrsp,32 488D6C2420 learbp,[rsp+20H]G_M1402_IG02:  33C0 xoreax,eax 85C0 testeax,eax7406je SHORT G_M1402_IG03 FF15FA7B7800 call[System.Console:WriteLine()]G_M1402_IG03: 90nopG_M1402_IG04:  4883C420 addrsp,32 5D poprbp C3 ret; Total bytes of code 29

now:

; Assembly listing for method P:Main()G_M1402_IG01: 55pushrbp 488BEC movrbp,rspG_M1402_IG02: G_M1402_IG03:  5D poprbp C3 ret; Total bytes of code 6

Two notes:

1. SplitCriticalEdges used to assume edges couldn't be folded away unless we optimize and instrument
2. CLFLG_CONSTANTFOLD is removed as not useful - there are tons of places where it's not checked anyway

[EgorBo]

/azp run runtime-coreclr outerloop, runtime-coreclr pgo, runtime-coreclr libraries-pgo

[azure-pipelines]

Azure Pipelines successfully started running 3 pipeline(s).

[EgorBo]

@AndyAyersMS@jakobbotsch PTAL

[AndyAyersMS]

We have an explicit Tier0 flag, why not check this?

[AndyAyersMS]

For the gtFold... seems like we should have a consistent pattern we use -- either the caller or the callee should check if optimizations are enabled, but not both?

Maybe this is covered in your opt levels PR?

[AndyAyersMS]

Linking to #9120.

[EgorBo]

@AndyAyersMS thanks! Merging to make it to Preview2. I'll indeed clean up the checks in #77465

[EgorBo]

Failure is #82397