[MachineLateInstrsCleanup] Minor fixing (NFC). (#117816)

With cb57b7a, MachineLateInstrsCleanup switched to using a map to keep
track of kill flags to remedy compile time regressions seen with huge
functions. It seems that the comment above clearKillsForDef() became stale with
that commit, and also that one of the arguments to it became unused,
both of which this patch fixes.

Author: JonPsson1

llvm/lib/CodeGen/MachineLateInstrsCleanup.cpp

@@ -57,7 +57,6 @@ class MachineLateInstrsCleanup : public MachineFunctionPass {
 
   void removeRedundantDef(MachineInstr *MI);
   void clearKillsForDef(Register Reg, MachineBasicBlock *MBB,
-                        MachineBasicBlock::iterator I,
                         BitVector &VisitedPreds);
 
 public:
@@ -110,14 +109,11 @@ bool MachineLateInstrsCleanup::runOnMachineFunction(MachineFunction &MF) {
   return Changed;
 }
 
-// Clear any previous kill flag on Reg found before I in MBB. Walk backwards
-// in MBB and if needed continue in predecessors until a use/def of Reg is
-// encountered. This seems to be faster in practice than tracking kill flags
-// in a map.
-void MachineLateInstrsCleanup::
-clearKillsForDef(Register Reg, MachineBasicBlock *MBB,
-                 MachineBasicBlock::iterator I,
-                 BitVector &VisitedPreds) {
+// Clear any preceding kill flag on Reg after removing a redundant
+// definition.
+void MachineLateInstrsCleanup::clearKillsForDef(Register Reg,
+                                                MachineBasicBlock *MBB,
+                                                BitVector &VisitedPreds) {
   VisitedPreds.set(MBB->getNumber());
 
   // Kill flag in MBB
@@ -137,13 +133,13 @@ clearKillsForDef(Register Reg, MachineBasicBlock *MBB,
   assert(!MBB->pred_empty() && "Predecessor def not found!");
   for (MachineBasicBlock *Pred : MBB->predecessors())
     if (!VisitedPreds.test(Pred->getNumber()))
-      clearKillsForDef(Reg, Pred, Pred->end(), VisitedPreds);
+      clearKillsForDef(Reg, Pred, VisitedPreds);
 }
 
 void MachineLateInstrsCleanup::removeRedundantDef(MachineInstr *MI) {
   Register Reg = MI->getOperand(0).getReg();
   BitVector VisitedPreds(MI->getMF()->getNumBlockIDs());
-  clearKillsForDef(Reg, MI->getParent(), MI->getIterator(), VisitedPreds);
+  clearKillsForDef(Reg, MI->getParent(), VisitedPreds);
   MI->eraseFromParent();
   ++NumRemoved;
 }