Fix a minor bug in two-address pass. It was missing a commute opportunity.

regB = move RCX
regA = op regB, regC
RAX  = move regA
where both regB and regC are killed. If regB is constrainted to non-compatible
physical registers but regC is not constrainted at all, then it's better to
commute the instruction.
       movl    %edi, %eax
       shlq    $32, %rcx
       leaq    (%rcx,%rax), %rax
=>
       movl    %edi, %eax
       shlq    $32, %rcx
       orq     %rcx, %rax
rdar://8762995

llvm-svn: 121793