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

[Manish Kausik H via llvm-commits]

Nirhar wrote:

> 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

@arsenm Yeah, I agree, I'll modify it. Just to be on the same note, the intention of this patch is to use unaligned move instructions for AVX registers when the function has the `no-stack-realign` attribute. 

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