diff --git a/llvm/lib/CodeGen/ModuloSchedule.cpp b/llvm/lib/CodeGen/ModuloSchedule.cpp
--- a/llvm/lib/CodeGen/ModuloSchedule.cpp
+++ b/llvm/lib/CodeGen/ModuloSchedule.cpp
@@ -1658,8 +1658,8 @@
     // we don't need the phi anymore.
     if (getStage(Def) == Stage) {
       Register PhiReg = MI.getOperand(0).getReg();
-      MRI.replaceRegWith(MI.getOperand(0).getReg(),
-                         Def->getOperand(0).getReg());
+      assert(Def->findRegisterDefOperandIdx(MI.getOperand(1).getReg()) != -1);
+      MRI.replaceRegWith(MI.getOperand(0).getReg(), MI.getOperand(1).getReg());
       MI.getOperand(0).setReg(PhiReg);
       PhiToDelete.push_back(&MI);
     }