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


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@121793 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/TwoAddressInstructionPass.cpp

@@ -554,7 +554,8 @@ TwoAddressInstructionPass::isProfitableToCommute(unsigned regB, unsigned regC,
   unsigned ToRegB = getMappedReg(regB, DstRegMap);
   unsigned ToRegC = getMappedReg(regC, DstRegMap);
   if (!regsAreCompatible(FromRegB, ToRegB, TRI) &&
-      (regsAreCompatible(FromRegB, ToRegC, TRI) ||
+      ((!FromRegC && !ToRegC) ||
+       regsAreCompatible(FromRegB, ToRegC, TRI) ||
        regsAreCompatible(FromRegC, ToRegB, TRI)))
     return true;