Fix OperIs(GT_CNS_INT, GT_CNS_LNG) assertion during import

[dhartglassMSFT]

Assertion in gtFoldExprHWIntrinsic on Vector.WithElement. The index argument could be a COMMA (boundscheck+integral constant) rather than just an integral constant as expected.

The windows-x64 antigen runs hit this assert pretty frequently, so fixing here to reduce the noise.

fixes #126060

[dotnet-policy-service]

Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch
See info in area-owners.md if you want to be subscribed.

[Copilot]

Pull request overview

Fixes a CoreCLR JIT assertion hit during import-time folding of Vector*.WithElement when the index operand is represented as a COMMA node (e.g., a boundscheck sequenced with a constant), and adds a regression test to ensure out-of-range behavior remains correct.

Changes:

• Add an additional operand-shape guard in gtFoldExprHWIntrinsic to only fold WithElement when the index operand is an actual integer constant node.
• Add a new JIT regression test covering Vector256.WithElement with an out-of-range constant index (verifies it throws ArgumentOutOfRangeException).
• Include the new regression test source file in the Regression_ro_2 test project.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/coreclr/jit/gentree.cpp	Prevents folding WithElement unless the index operand is a constant int node, avoiding an assertion when the index is a COMMA tree.
src/tests/JIT/Regression/Regression_ro_2.csproj	Adds the new regression test to the compilation list.
src/tests/JIT/Regression/JitBlue/Runtime_126060/Runtime_126060.cs	New regression validating Vector256.WithElement throws on out-of-range index and does not trigger the JIT folding assertion.