JIT: Only optimize out integral casts for returned fields

[jakobbotsch]

Fix #118566

[Copilot]

Pull Request Overview

This PR fixes a bug in the JIT's optimization of integral casts for returned fields by adding an additional constraint to ensure the source type is also an integral register type.

• Adds a check to verify that the cast source type (CastFromType()) uses an integral register before optimizing out the cast
• This prevents incorrect optimization when casting from non-integral types (like floating-point) to integral types

Comments suppressed due to low confidence (1)

src/coreclr/jit/lower.cpp:5323

• [nitpick] The fix appears correct but the condition order could be more efficient. Consider moving varTypeUsesIntReg(node->CastFromType()) before the size comparison since type checks are typically faster than size calculations and can short-circuit the evaluation earlier.

                   varTypeUsesIntReg(node->CastFromType()) && (genTypeSize(regType) <= genTypeSize(node->CastToType())))

[jakobbotsch]

cc @dotnet/jit-contrib PTAL @EgorBo

Regression introduced by #118345.

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[EgorBo]

Just curious - is this some recent regression? Ah, I see the link