Index: llvm/lib/Target/X86/X86FlagsCopyLowering.cpp
===================================================================
--- llvm/lib/Target/X86/X86FlagsCopyLowering.cpp
+++ llvm/lib/Target/X86/X86FlagsCopyLowering.cpp
@@ -727,8 +727,16 @@
   if (!CondReg)
     CondReg = promoteCondToReg(TestMBB, TestPos, TestLoc, Cond);
 
-  // Rewriting this is trivial: we just replace the register and remove the
-  // setcc.
-  MRI->replaceRegWith(SetCCI.getOperand(0).getReg(), CondReg);
-  SetCCI.eraseFromParent();
+  // Rewriting a register def is trivial: we just replace the register and
+  // remove the setcc.
+  MachineOperand &Op0 = SetCCI.getOperand(0);
+  if (Op0.isDef()) {
+    assert(Op0.isReg() &&
+           "Cannot have a non-register defined operand to SETcc!");
+    MRI->replaceRegWith(SetCCI.getOperand(0).getReg(), CondReg);
+    SetCCI.eraseFromParent();
+    return;
+  }
+
+  llvm_unreachable("boom");
 }
Index: llvm/test/CodeGen/X86/flags-copy-lowering.mir
===================================================================
--- llvm/test/CodeGen/X86/flags-copy-lowering.mir
+++ llvm/test/CodeGen/X86/flags-copy-lowering.mir
@@ -208,11 +208,10 @@
     %3:gr8 = SETAr implicit $eflags
     %4:gr8 = SETBr implicit $eflags
     %5:gr8 = SETEr implicit $eflags
-    %6:gr8 = SETNEr implicit killed $eflags
+    SETNEm $rsp, 1, $noreg, -16, $noreg, implicit killed $eflags
     MOV8mr $rsp, 1, $noreg, -16, $noreg, killed %3
     MOV8mr $rsp, 1, $noreg, -16, $noreg, killed %4
     MOV8mr $rsp, 1, $noreg, -16, $noreg, killed %5
-    MOV8mr $rsp, 1, $noreg, -16, $noreg, killed %6
   ; CHECK-NOT:     $eflags =
   ; CHECK-NOT:             = SET{{.*}}
   ; CHECK:         MOV8mr {{.*}}, killed %[[A_REG]]