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

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

LGTM but It took me a minute to realize what this was doing. The title is misleading. This is not unconditionally using unaligned store, it is avoiding introducing stack realignments 

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