Generate better two address code by using commutativity

src/IceTargetLoweringX86BaseImpl.h

Index: src/IceTargetLoweringX86BaseImpl.h
diff --git a/src/IceTargetLoweringX86BaseImpl.h b/src/IceTargetLoweringX86BaseImpl.h
index ebdfb1e86b7e94dc86c2ad4f413da5dc754a41a0..d06474c1a602ab347fa66cb1d088467e93017ed9 100644
--- a/src/IceTargetLoweringX86BaseImpl.h
+++ b/src/IceTargetLoweringX86BaseImpl.h
@@ -1283,6 +1283,11 @@ void TargetX86Base<Machine>::lowerArithmetic(const InstArithmetic *Inst) {
       std::swap(Src0, Src1);
     if (llvm::isa<Constant>(Src0) && !llvm::isa<Constant>(Src1))
       std::swap(Src0, Src1);
+    // Improve two-address code patterns by avoiding a copy to the dest
+    // register when one of the source operands ends its lifetime here.
+    if (!Inst->isLastUse(Src0) && Inst->isLastUse(Src1))
+      std::swap(Src0, Src1);

[Jim Stichnoth, 2015/09/25 21:12:09]

I'm starting to get nervous about "swap wars".  Could you add code to count how
many swaps are performed, and then assert that the count is at most 1?

Perhaps the cleaner alternative is to use "else if" instead of "if", but I like
the counting approach which may catch unexpected behavior in the future.

[sehr, 2015/10/06 18:42:00]

I added the counter.

+
   }
   if (!Traits::Is64Bit && Dest->getType() == IceType_i64) {
     // These x86-32 helper-call-involved instructions are lowered in this