[PowerPC] Remove self-copies in pre-emit peephole

There are occasionally instances where AADB rewrites registers in such a way
that a reg-reg copy becomes a self-copy. Such an instruction is obviously
redundant and can be removed. This patch does precisely that.

Note that this will not remove various nop's that we insert (which are
themselves just self-copies). The reason those are left alone is that all of
them have their own opcodes (that just encode to a self-copy).

What prompted this patch is the fact that these self-copies sometimes end up
using registers that make the instruction a priority-setting nop, thereby
having a significant effect on performance.

Differential revision: https://reviews.llvm.org/D52432


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@344036 91177308-0d34-0410-b5e6-96231b3b80d8