[PowerPC] Support extended mnemonics in AsmParser

This patch adds infrastructure to support extended mnemonics in the
PowerPC assembler parser.  It adds support specifically for those
extended mnemonics that LLVM will itself generate.

The test case lists *all* extended mnemonics according to the
PowerPC ISA v2.06 Book I, but marks those not yet supported
as FIXME.

llvm-svn: 181051