AArch64: error when both positional & named operands are used.

Only one instruction pair needed changing: SMULH & UMULH. The previous
code worked, but MC was doing extra work treating Ra as a valid
operand (which then got completely overwritten in MCCodeEmitter).

No behaviour change, so no tests.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203772 91177308-0d34-0410-b5e6-96231b3b80d8