[X86] `X86TargetLowering`: override `allowsMemoryAccess()`

The baseline `allowsMemoryAccess()` is wrong for X86.
It assumes that aligned memory operations are always allowed,
but that is not true.

For example, We can not perform a 32-byte aligned non-temporal load
of a 32-byte vector, without AVX2 that is, yet `allowsMemoryAccess()`
will say it is allowed, so we may end up merging non-temporal loads,
only to split them up to legalize them, and here we go again.

NOTE: the test changes here are superfluous. The main effect is that without this change,
in D141777, we'd get stuck endlessly merging and splitting non-temporal stores.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D141776