[CGP] Update MemIntrinsic alignment if possible

Previously it was only being done if shouldAlignPointerArgs() returned
true, which right now is only true for ARM targets.
Updating the argument alignment attributes of memcpy/memset intrinsics
if the underlying object has larger alignment can be beneficial even
when CGP didn't increase alignment (as can be seen from the test changes),
so invert the loop and if condition.

Differential Revision: https://reviews.llvm.org/D134281