Fix for out of range rotate amount during lower

[dhartglassMSFT]

Assert during arm64 lower that rotate amount on a rotate node (ROR, ROL) was an out of range integer

Change to instead add explicit masking onto rotate nodes in the jit (AND op (CNS_INT 63) for ex.), and to strip the mask during lower like is done already for shifts

Also change importercalls to allow non-constant rotate amounts onto the node. Gives a couple nice diffs where we can generate ROR now instead of a call.

There's a single bad diff caused by CSE deduplicating the explicit masks, which ends up defeating the lowering mask-stripping logic. This bug exists already for SHIFTS (not rotates) so this is an extant issue, I'll try and open a bug.

fixes #129298

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[Copilot]

Pull request overview

This PR updates the JIT’s handling of rotate counts to avoid out-of-range rotate amounts reaching lowering, including enabling import of variable rotate amounts for primitive intrinsics, and adds a regression test for the arm64 lowering assert scenario.

Changes:

• Mask rotate counts explicitly in morphing so rotate nodes always have counts normalized to [0, bitsize-1].
• Strip redundant AND(count, mask) for rotates during lowering on targets that implicitly mask rotate counts.
• Refactor RotateLeft/RotateRight intrinsic importing into a shared helper and allow non-constant rotate amounts (with explicit masking), plus add a regression test.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/coreclr/jit/morph.cpp	Adds explicit masking of rotate counts when recognizing/morphing rotation patterns.
src/coreclr/jit/lower.cpp	Adds TryRemoveShiftRotateMask and uses it to strip redundant rotate-count masks prior to lowering rotates on relevant targets.
src/coreclr/jit/lower.h	Declares TryRemoveShiftRotateMask helper used by lowering.
src/coreclr/jit/importercalls.cpp	Introduces impRotateHelper and uses it for RotateLeft/RotateRight, including masked variable rotate counts.
src/coreclr/jit/compiler.h	Adds the impRotateHelper declaration.
src/tests/JIT/Regression/JitBlue/Runtime_129298/Runtime_129298.cs	Adds regression test ensuring JIT compiles the dead rotate block without asserting.
src/tests/JIT/Regression/Regression_ro_2.csproj	Includes the new regression test source file in the project.

[Copilot]

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated no new comments.

Suppressed comments (1)

src/coreclr/jit/lower.cpp:8875

• The LowerShift doc comment still refers specifically to xarch, but this helper is used for multiple targets (see the TARGET_XARCH || TARGET_ARM64 || TARGET_LOONGARCH64 || TARGET_RISCV64 call sites). Updating the comment would avoid misleading future readers about why the mask removal is safe here.

// Notes:
//    Remove unnecessary shift count masking, xarch shift instructions
//    mask the shift count to 5 bits (or 6 bits for 64 bit operations).
//