Liveness fix for struct enreg.

[sandreenko]

Delete some unnecessary mustInit. The change is required for a small struct enreg change.

spmi changes:

benchmarks.run.windows.x64.checked.mch
Total bytes of delta: -725 (-1.10% of base)
71 total files with Code Size differences (69 improved, 2 regressed), 0 unchanged.

libraries.crossgen.windows.x64.checked
Total bytes of delta: -479 (-0.37% of base)
42 total files with Code Size differences (41 improved, 1 regressed), 0 unchanged.

libraries.crossgen2.windows.x64.checked.mch
Total bytes of delta: -3296 (-0.90% of base)
373 total files with Code Size differences (345 improved, 28 regressed), 8 unchanged.

libraries.pmi.windows.x64.checked
Total bytes of delta: -2101 (-0.62% of base)
243 total files with Code Size differences (214 improved, 29 regressed), 0 unchanged.

tests.pmi.windows.x64.checked
Total bytes of delta: -18637 (-0.81% of base)
1475 total files with Code Size differences (1472 improved, 3 regressed), 2 unchanged.

[sandreenko]

/azp run runtime-coreclr gcstress0x3-gcstress0xc

[sandreenko]

#51900

[sandreenko]

There are many failures but they all look unrelated, the same gcstress failures happened in https://dev.azure.com/dnceng/public/_build/results?buildId=1095005&view=ms.vss-test-web.build-test-results-tab, the jitstress failure presented in the previous run as well.

[sandreenko]

PTAL @BruceForstall @dotnet/jit-contrib , cc @erozenfeld

[erozenfeld]

The new logic doesn't take into account varDsc->lvOnFrame. Is that intentional?

[sandreenko]

Good question, if I add an assert like if (mustInitThisVar) assert(varDsc->lvOnFrame); it won't fail (x64 spmi) but it is a current implicit contract so we probably should not rely on it.

if (hasGCPtr && !isTracked) -> not tracked means not in register -> lvOnFrame == true
else if (hasGCPtr && isStruct) -> do not enregister structs, with https://github.com/dotnet/runtime/pull/51850/checks?check_run_id=2447290001 won't enregister structs with GC pointers -> lvOnFrame == true
if (!isTracked)  -> not tracked means not in register -> lvOnFrame == true

I will return the condition, thank you.