[PowerPC] Fix load/store selection infrastructure when load/store intrinsics are used on P10.

The load/store infrastructure previously made an incorrect assumption that
whenever it is used with a load/store intrinsic on Power10 - those intrinsics
would automatically be the lxvp/stxvp intrinsics introduced in Power10.

However, this is obviously not the case as there are multiple instances of
pre-P10 intrinsics that use the refactored load/store implementation.
This patch corrects this assumption, and produces the expected intrinsic on pre-P10.

Differential Revision: https://reviews.llvm.org/D114978