[llvm] [GlobalISel] Preserve volatile and ordered atomic stores of undef (PR #226265)

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<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-llvm-globalisel

@llvm/pr-subscribers-backend-aarch64

Author: Alex MacLean (AlexMaclean)

<details>
<summary>Changes</summary>

Guard `erase_undef_store` with `isUnordered()`, matching InstCombine. Ensures volatile and ordered atomic stores are preserved.

https://github.com/llvm/llvm-project/blob/e27c7439ab588237a4c526feb3bb632f9aea8530/llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp#L1489-L1491

Co-authored-by: Hongyu Chen <hongchen@<!-- -->nvidia.com>


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Full diff: https://github.com/llvm/llvm-project/pull/226265.diff


2 Files Affected:

- (modified) llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp (+2) 
- (modified) llvm/test/CodeGen/AArch64/GlobalISel/prelegalizercombiner-undef.mir (+114) 


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</details>


https://github.com/llvm/llvm-project/pull/226265