[WIP] Fix GC hole in JIT due to if-conversion

[Copilot]

• Inspect explicit-init dependencies and complete the baseline build.
• Remove explicit-init flags and dependent logic; add focused initialization coverage.
• Build, run targeted tests, and check codegen impact: host/cross-target Debug JIT builds and six Linux x64 test runners passed; the normal regression wrapper fails on baseline and passes with the fix. Focused reference initialization adds 2 instructions / 6 bytes; existing large-struct zeroing codegen is unchanged. The crashing GC-stress repro was not run.
• Complete formatting, code review, and security validation: clang-format 17 and secret scans passed; manual review feedback addressed. Full jit-format and the automated review tool were unavailable; CodeQL analysis was skipped because of database-size limits.
• Resolves JIT: (bug) GC hole. If-conversion sinks the explicit init of an untracked GC local past a safepoint, leaving an uninitialized reported GC slot #133754

• Fixes JIT: (bug) GC hole. If-conversion sinks the explicit init of an untracked GC local past a safepoint, leaving an uninitialized reported GC slot #133754

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[EgorBo]

@copilot remove changes to StructWithGC_Zeroing tests, no tests needed.

[Copilot]

@copilot remove changes to StructWithGC_Zeroing tests, no tests needed.

Reverted both test files to their pre-PR versions in 735995d. The JIT fix is unchanged.

[EgorBo]

@jakobbotsch @AndyAyersMS @dotnet/jit-contrib so this PR removes hasExplicitInit entirely (with the flag) - see PR changes with "ignore whitespaces" mode.

Diffs - not that bad for non-tests collections, but still a regression. @jakobbotsch pointed out that the current change is not correct - it assumes at the SSA liveness stage we know exactly what parts of code are no-gc areas which is not correct, so presumably there might be a scenario when we have a GC local, it's not zeroed yet and we hit an unexpected gc safe point (e.g. fully interruptible part) - it will crash. Should we take it as is given it's a correctness fix?

I prototyped a fix that does the same analysis at the codegen phase that recovers the regressions, but it was a bit too complex for this PR IMO.

[EgorBo]

It seems that regression happen moslty in small methods where the existing opt managed to avoid any zeroing in prolog, e.g.

nuint Test(ref int value)
{
    fixed (int* p = &value)
        return (nuint)p & 1;
}

Main:

; Method WriteBarrierBenchmarks:Test(byref):nuint (FullOpts)
G_M49291_IG01:
       push     rax
G_M49291_IG02:
       mov      bword ptr [rsp], rcx
       mov      rax, rcx
       and      rax, 1
G_M49291_IG03:
       add      rsp, 8
       ret      
; Total bytes of code: 17

PR:

; Method WriteBarrierBenchmarks:Test(byref):nuint (FullOpts)
G_M49291_IG01:
       push     rax
       xor      eax, eax
       mov      qword ptr [rsp], rax
G_M49291_IG02:
       mov      bword ptr [rsp], rcx
       mov      rax, rcx
       and      rax, 1
G_M49291_IG03:
       add      rsp, 8
       ret      
; Total bytes of code: 23

(in the real world, it's likely inlined to somewhere where we already zero locals with SIMD anyway)

[AndyAyersMS]

I'm a bit worried about the potential impact of this change. Wondering if we should consider something more surgical.

Is this a regression in .NET 11?