[llvm] [SelectionDAG] Use unaligned store to move AVX registers onto stack for `extractelement` (PR #78422)

[Manish Kausik H via llvm-commits]

Nirhar wrote:

@RKSimon I regenerated the test file `sve-extract-fixed-vector.ll` with update-llc-test-checks.py to accomodate new changes.

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