fix: expand GT_UMOD with constant divisor in morph to enable CSE

[mad1081]

Fixes #119131

Related:

• JIT: Avoid reordering operands in fgMorphModToSubMulDiv #71615 — fgMorphModToSubMulDiv was already made to handle GT_UMOD correctly
• [JIT] Support x % {pow2_cns} transformation for ulong types #79676 — unsigned mod support was extended the same way before

On XARCH, fgMorphSmpOp expands a % b into a - (a / b) * b during morph so that CSE can recognize a shared divisor between a division and a modulo with the same operands. This expansion was gated on tree->OperIs(GT_MOD), so unsigned modulo (GT_UMOD) with a non-power-of-2 constant divisor was never expanded in morph — it was
left for lowering instead. Since CSE runs before lowering, patterns like:

uint arrIndex = index / 3;
uint bitIndex = index % 3;

produced two div instructions instead of one.

fgMorphModToSubMulDiv already handles GT_UMOD correctly (it switches the operator to GT_UDIV and calls CheckDivideByConstOptimized), so the fix is to include GT_UMOD in the condition. Power-of-2 unsigned divisors are unaffected — they are already handled earlier by fgMorphUModToAndSub.

Note: ulong UMOD on XARCH is still blocked by a separate early return that predates this fix and is not addressed here.

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[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Updates XARCH JIT morphing to expand unsigned modulo (GT_UMOD) by a constant divisor into a - (a / b) * b earlier (during morph) so CSE can reuse a shared division when both / and % appear with the same operands.

Changes:

• Extend the morph-time mod expansion gate from GT_MOD to GT_MOD and GT_UMOD for non-power-of-2 constant divisors.
• Update the surrounding comment to reflect unsigned modulo handling and the CSE motivation.