[PowerPC] Fix the register allocation hints for ACC registers.

The allocation hints for copies of ACC registers assumed that we would only be
copying between VSRp and UACC registers. In reality it is also possible to copy
between UACC and ACC registers.

This patch adds a new case for the ACC copy to fix that issue.
Note that the test case added with this patch will hit an assert without the
fix.

Reviewed By: lei, amyk

Differential Revision: https://reviews.llvm.org/D134501