[ARM][RDA] Allow multiple killed users

In RDA, check against the already decided dead instructions when
looking at users. This allows an instruction to be removed if it
has multiple users, but they're all dead.

This means that IT instructions can be considered killed once all
the itstate using instructions are dead.

Differential Revision: https://reviews.llvm.org/D75245

Author: sparker-arm

llvm/lib/CodeGen/ReachingDefAnalysis.cpp

@@ -547,7 +547,7 @@ ReachingDefAnalysis::isSafeToRemove(MachineInstr *MI, InstSet &Visited,
 void ReachingDefAnalysis::collectLocalKilledOperands(MachineInstr *MI,
                                                      InstSet &Dead) const {
   Dead.insert(MI);
-  auto IsDead = [this](MachineInstr *Def, int PhysReg) {
+  auto IsDead = [this, &Dead](MachineInstr *Def, int PhysReg) {
     unsigned LiveDefs = 0;
     for (auto &MO : Def->operands()) {
       if (!isValidRegDef(MO))
@@ -561,7 +561,10 @@ void ReachingDefAnalysis::collectLocalKilledOperands(MachineInstr *MI,
 
     SmallPtrSet<MachineInstr*, 4> Uses;
     getGlobalUses(Def, PhysReg, Uses);
-    return Uses.size() == 1;
+    for (auto *Use : Uses)
+      if (!Dead.count(Use))
+        return false;
+    return true;
   };
 
   for (auto &MO : MI->operands()) {

llvm/lib/Target/ARM/ARMLowOverheadLoops.cpp

@@ -943,11 +943,8 @@ void ARMLowOverheadLoops::IterationCountDCE(LowOverheadLoop &LoLoop) {
   if (ModifiedITs.empty()) {
     LLVM_DEBUG(dbgs() << "ARM Loops: Will remove iteration count:\n";
                for (auto *MI : Killed)
-                 dbgs() << " - " << *MI;
-               for (auto *MI : DeadITs)
                  dbgs() << " - " << *MI);
     LoLoop.ToRemove.insert(Killed.begin(), Killed.end());
-    LoLoop.ToRemove.insert(DeadITs.begin(), DeadITs.end());
   }
 
   // Collect and remove the users of iteration count.