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
