[PowerPC] Add peephole to remove redundant accumulator prime/unprime instructions

In some situations, the compiler may insert an accumulator prime instruction and
an accumulator unprime instruction with no use of that accumulator between the two.
That's for example the case when we store an accumulator after assembling it or
restoring it. This patch adds a peephole to remove these prime and unprime instructions.

Differential Revision: https://reviews.llvm.org/D91386