[llvm] [SelectionDAG] Use unaligned store/load to move AVX registers onto stack for `insertelement` (PR #82130)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

ExpandInsertToVectorThroughStack is for INSERT_SUBVECTOR not INSERT_VECTOR_ELT, but I agree we can probably share more of the code between them.

https://github.com/llvm/llvm-project/pull/82130