ScheduleDAGInstrs: Do not try to toggle kill flags on debug uses

Preparation for upcoming changes. No testcase as none of the public
targets bundles early enough and has a post machine scheduler enabled at
the same time. The error is also easily catched by asserts.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@293324 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/ScheduleDAGInstrs.cpp

@@ -1159,6 +1159,9 @@ static void toggleBundleKillFlag(MachineInstr *MI, unsigned Reg,
 }
 
 void ScheduleDAGInstrs::toggleKillFlag(MachineInstr &MI, MachineOperand &MO) {
+  if (MO.isDebug())
+    return;
+
   // Setting kill flag...
   if (!MO.isKill()) {
     MO.setIsKill(true);