fix a subtle bug I introduced in my refactoring, where we stopped preferring

the i8 versions of instructions in some cases.  In test6, we started 
generating:

	cmpq	$0, -8(%rsp)            ## encoding: [0x48,0x81,0x7c,0x24,0xf8,0x00,0x00,0x00,0x00]
                                        ## <MCInst #478 CMP64mi32
                                        ##  <MCOperand Reg:114>
                                        ##  <MCOperand Imm:1>
                                        ##  <MCOperand Reg:0>
                                        ##  <MCOperand Imm:-8>
                                        ##  <MCOperand Reg:0>
                                        ##  <MCOperand Imm:0>>

instead of:

	cmpq	$0, -8(%rsp)            ## encoding: [0x48,0x83,0x7c,0x24,0xf8,0x00]
                                        ## <MCInst #479 CMP64mi8
                                        ##  <MCOperand Reg:114>
                                        ##  <MCOperand Imm:1>
                                        ##  <MCOperand Reg:0>
                                        ##  <MCOperand Imm:-8>
                                        ##  <MCOperand Reg:0>
                                        ##  <MCOperand Imm:0>>

Fix this and add some comments.

llvm-svn: 116053