[llvm] [SelectionDAG] Use unaligned store to move AVX registers onto stack for `extractelement` (PR #78422)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon requested changes to this pull request.

Please can you check the build failure in CodeGen/AArch64/sve-extract-fixed-vector.ll

https://github.com/llvm/llvm-project/pull/78422