[PowerPC] Relax register superclasses for paired memops

Relaxing superclass constraint for VSX register classes helps reducing
32-byte spills and copies when register pressure is high.

In test case affected, some of them introduces more copies due to new
allocation order. However, this patch should not be the root cause, and
we may be able to fix it in other places of register allocation.

Reviewed By: nemanjai

Differential Revision: https://reviews.llvm.org/D104006