[x86] enhance mayFoldLoad to check alignment

As noted in D112464, a pre-AVX target may not be able to fold an
under-aligned vector load into another op, so we shouldn't report
that as a load folding candidate. I only found one caller where
this would make a difference -- combineCommutableSHUFP() -- so
that's where I added a test to show the (minor) regression.

Differential Revision: https://reviews.llvm.org/D112545