Use MCPhysReg for table of register encodings.
Teach disassembler to handle illegal immediates on (v)cmpps/pd/ss/sd instructions. Instead of rejecting we'll just generate the _alt forms that don't try to alter the mnemonic. While I'm here, merge some common code in the Instruction printers for the condition code replacement and fix the mask on SSE to be 3-bits instead of 4.