[AggressiveInstCombine] Use AA during store merge (#149992)
This is a small extension of #147540, resolving one of the FIXMEs.
Instead of bailing out on any instruction that may read/write memory,
use AA to check whether it can alias the stored parts. Do this using a
crude check based on the underlying object only.
This pattern occurs rarely in practice, but at the same time it also doesn't
seem to add any compile-time cost, so it's probably worth handling.