[llvm] [clang] [clang-tools-extra] [X86] Use plain load/store instead of cmpxchg16b for atomics with AVX (PR #74275)

[Simon Pilgrim via cfe-commits]

https://github.com/RKSimon approved this pull request.

LGTM. I'd still like to ensure we have unaligned x86 test coverage.

https://github.com/llvm/llvm-project/pull/74275