[PowerPC] emit VSX instructions instead of VMX instructions for vector loads and stores

This patch changes the PowerPC backend to generate VSX load/store instructions
for all vector loads/stores on Power8 and earlier  (LE) instead of VMX
load/store instructions. The reason for this change is because VMX instructions
require the vector to be 16-byte aligned. So, a vector load/store will fail with
VMX instructions if the vector is misaligned. Also, `gcc` generates VSX
instructions in this situation which allow for unaligned access but require a
swap instruction after loading/before storing. This is not an issue for BE
because we already emit VSX instructions since no swap is required. And this is
not an issue on Power9 and up since we have access to `lxv[x]`/`stxv[x]` which
allow for unaligned access and do not require swaps.

This patch also delays the VSX load/store for LE combines until after
LegalizeOps to prioritize other load/store combines.

Reviewed By: #powerpc, stefanp

Differential Revision: https://reviews.llvm.org/D127309