[SimplifyCFG] Check alignment when speculating stores

When speculating a store based on a preceding load/store, we need
to ensure that the speculated store does not have a higher
alignment (which might only be guaranteed by the branch condition).

There are various ways in which this could be strengthened (we
could get or enforce the alignment), but for now just do the
simple check against the preceding load/store.

Fixes https://github.com/llvm/llvm-project/issues/89672.