ARM Assembly syntax support for arithmetic implied dest operand.

ARM Assembly syntax support for arithmetic implied dest operand.

When the destination operand is the same as the first source register
operand for arithmetic instructions, the destination operand may be omitted.

For example, the following two instructions are equivalent:
  sub r2, r2, #6
  sub r2, #6

rdar://9682597

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@133925 91177308-0d34-0410-b5e6-96231b3b80d8