JIT: Preserve evaluation order in GC bulk copies

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GC bulk-copy lowering can fault on the destination before evaluating the source, skipping side effects or masking source exceptions. In Unsafe.Add(ref dst, Index()) = GetSrc(), GetSrc() must run before the copy faults.

• Lowering: Defer both null checks until both addresses are evaluated. Move GT_PUTARG_* nodes afterward so argument placement remains adjacent to the helper call.
• Regression coverage: Add six cases covering valid/null destinations and valid/null/throwing sources, asserting evaluation order, exception types, and copied references.

• Fixes JIT: (bug) Struct copy through the GC bulk-copy helper faults on the destination before the source expression is evaluated #133587

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Pull request overview

This pull request preserves evaluation order in JIT GC bulk-copy lowering and adds regression coverage.

Changes:

• Defers null checks until both addresses are evaluated.
• Keeps argument placement adjacent to the helper call.
• Adds six evaluation-order and exception regression cases.

File summaries

File	Summary
src/tests/JIT/Regression_ro_2/Runtime_133587.cs	Adds regression tests for evaluation order and copy behavior.
src/coreclr/jit/lower.cpp	Adjusts GC bulk-copy lowering order.

Review details

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

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