PeepholeOpt: Fix copy current source index accounting bug

We were essentially using the current source index as a binary
value, and didn't actually use it for indexing so it did not
matter. Use the operand to ensure the value is actually correct.

Author: arsenm

llvm/lib/CodeGen/PeepholeOptimizer.cpp

@@ -201,11 +201,11 @@ class CopyRewriter : public Rewriter {
 
   bool getNextRewritableSource(RegSubRegPair &Src,
                                RegSubRegPair &Dst) override {
-    if (CurrentSrcIdx++ > 1)
+    if (++CurrentSrcIdx > 1)
       return false;
 
     // The rewritable source is the argument.
-    const MachineOperand &MOSrc = CopyLike.getOperand(1);
+    const MachineOperand &MOSrc = CopyLike.getOperand(CurrentSrcIdx);
     Src = RegSubRegPair(MOSrc.getReg(), MOSrc.getSubReg());
     // What we track are the alternative sources of the definition.
     const MachineOperand &MODef = CopyLike.getOperand(0);