[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.

As @davemgreen suggested - please add basic handling for scalable vector types.

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