hook up getOpcodeName for ARM so that "llc -show-mc-inst" includes

the opcode string in the inst dump, e.g.:
	vmov	r2, r3, d17             @ encoding: [0x31,0x2b,0x53,0xec]
                                        @ <MCInst #989 VMOVRRD
                                        @  <MCOperand Reg:68>
                                        @  <MCOperand Reg:69>
                                        @  <MCOperand Reg:19>
                                        @  <MCOperand Imm:14>
                                        @  <MCOperand Reg:0>>

The "VMOVRRD" is new.

llvm-svn: 117609