[AA] Improve precision for monotonic atomic load/store operations (#158169) Refines `getModRefInfo` for `Monotonic` and `Unordered` atomic operations to perform a full alias evaluation instead of conservatively returning `ModRef`. This allows AA to return more precise results (e.g., `NoModRef`), unblocking optimizations in passes like DSE/PRE when the pointers are proven not to alias. LIT testing: 1. Fixed the `XFAIL` test `@test9` in `DSE/atomic-todo.ll`, where the improved AA now allows DSE to correctly remove a dead store. 2. Added test `@test9a` to the same file to ensure DSE is still blocked when pointers may alias. 3. I am also reviewing tests in GVN/PRE to see where this change may be beneficial. While all existing tests pass, I will update this PR with any new GVN test cases in a follow-up commit. --------- Co-authored-by: Jin Huang <jingold@google.com>
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