[PowerPC] Use assembler source in MC tests

A couple of old test cases in test/MC/PowerPC were still using
LLVM IR.  Now that we have a working assembler, we can move
them to assembler tests instead:
  ppc64-initial-cfa.ll
  ppc64-relocs-01.ll
  ppc64-tls-relocs-01.ll

llvm-svn: 183829