add support for encoding the lo14 forms used for a few PPC64 addressing

modes.  For example, we now get:

	ld r3, lo16(_G)(r3)             ; encoding: [0xe8,0x63,A,0bAAAAAA00]
                                        ;   fixup A - offset: 0, value: lo16(_G), kind: fixup_ppc_lo14

llvm-svn: 119133