[PowerPC] Support some miscellaneous mnemonics in the asm parser

This adds support for the following extended mnemonics:
  xnop
  mr.
  not
  not.
  la



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184767 91177308-0d34-0410-b5e6-96231b3b80d8