Fix Enum.Equals folding for nullable boxed arguments

[Copilot]

Boxing a nullable enum can yield null while retaining an exact enum type. Enum.Equals folding currently dereferences that argument, throwing instead of returning false.

• JIT: Require a known non-null argument before comparing boxed payloads; otherwise retain the call. Preserve null-receiver exception behavior.
• Coverage: Add nullable-boxing cases for null, equal, and unequal arguments.

• Fixes JIT: (bug) Enum.Equals folding throws NullReferenceException instead of returning false for a null argument #133817

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[EgorBo]

@copilot fix ci failures

[Copilot]

@copilot fix ci failures

Fixed in e2f6aba. Enum-valued [InlineData] broke assembly aliases in the merged runner; all four cases now run from TestEntryPoint. Reproduced the CI compile failure, then rebuilt JIT_others successfully and passed enum coverage in four JIT modes. The other failures in that CI run are known issues #133798 and #133645.

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[Copilot]

🟢 Approval recommended

The fix is narrowly scoped and includes regression coverage with no unresolved review comments.

Pull request overview

Fixes JIT Enum.Equals folding for nullable enum values that box to null.

Changes:

• Guard payload comparison until the argument is known non-null.
• Add nullable-boxing regression tests.

File summaries

File	Description
src/tests/JIT/Intrinsics/EnumIntrinsics.cs	Adds null, equal, and unequal nullable-boxing coverage.
src/coreclr/jit/gentree.cpp	Prevents unsafe folding of nullable boxed arguments.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Lite

[EgorBo]

A few diffs

PTAL @jakobbotsch @dotnet/jit-contrib

[EgorBo]

ping @jakobbotsch, presumably, simple change

[EgorBo]

/ba-g DIC failure is already fixed in main