[LegalizeDAG] Propagate alignment in ExpandExtractFromVectorThroughStack

Unlike the name suggests this can reuse any store as a base for a
memory-based vector extract. If that store is underaligned the loads
created to extract will have an invalid alignment. Since most CPUs are
forgiving wrt alignment this is almost never an issue, on x86 this is
only reproducible by extracting a 128 bit vector out of a wider vector.

I tried making a test case in the context of
https://reviews.llvm.org/D127982 but it's really really fragile, as the
output pretty much looks like a missed optimization.