split out an encoder for memri operands, allowing a relocation to be plopped

into the immediate field.  This allows us to encode stuff like this:

        lbz r3, lo16(__ZL4init)(r4)     ; globalopt.cpp:5
                                        ; encoding: [0x88,0x64,A,A]
                                        ;   fixup A - offset: 0, value: lo16(__ZL4init), kind: fixup_ppc_lo16

        stw r3, lo16(__ZL1s)(r5)        ; globalopt.cpp:6
                                        ; encoding: [0x90,0x65,A,A]
                                        ;   fixup A - offset: 0, value: lo16(__ZL1s), kind: fixup_ppc_lo16

With this, we should have a completely function MCCodeEmitter for PPC, wewt.

llvm-svn: 119134