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

[Manish Kausik H via llvm-commits]

Nirhar wrote:

We seem to have 2 different functions that essentially do the same job of spilling the vector onto the stack for inserting into it, which are: 
https://github.com/llvm/llvm-project/blob/f81f7a57664bd7dccd5e179ed01155efbc8ff481/llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp#L125
and
https://github.com/llvm/llvm-project/blob/f81f7a57664bd7dccd5e179ed01155efbc8ff481/llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp#L193

As a suggestion for a different PR, I think we can get rid of one of them, or call one through the other.

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