add basic encoding support for immediates and registers, allowing us

to encode all of these instructions correctly (for example):

        mflr r0                         ; encoding: [0x7c,0x08,0x02,0xa6]
        stw r0, 8(r1)                   ; encoding: [0x90,0x01,0x00,0x08]
        stwu r1, -64(r1)                ; encoding: [0x94,0x21,0xff,0xc0]

llvm-svn: 119118