[ARM] Use DenseSet instead of DenseMap. NFC

The value in the map is set to "true" when something is added to
the map.

Techncally this:

  if (!DefRegs.contains(Reg))

will set the value in the map to false if it didn't alrady exist,
and this is used to indicate the value wasn't in the map. This only
occurs after all the "true" values have already been added to the map.

Author: topperc

llvm/lib/Target/ARM/ARMISelLowering.cpp

@@ -11366,12 +11366,12 @@ void ARMTargetLowering::EmitSjLjDispatchBlock(MachineInstr &MI,
            II = BB->rbegin(), IE = BB->rend(); II != IE; ++II) {
       if (!II->isCall()) continue;
 
-      DenseMap<unsigned, bool> DefRegs;
+      DenseSet<unsigned> DefRegs;
       for (MachineInstr::mop_iterator
              OI = II->operands_begin(), OE = II->operands_end();
            OI != OE; ++OI) {
         if (!OI->isReg()) continue;
-        DefRegs[OI->getReg()] = true;
+        DefRegs.insert(OI->getReg());
       }
 
       MachineInstrBuilder MIB(*MF, &*II);
@@ -11386,7 +11386,7 @@ void ARMTargetLowering::EmitSjLjDispatchBlock(MachineInstr &MI,
           continue;
         if (!Subtarget->isThumb() && !ARM::GPRRegClass.contains(Reg))
           continue;
-        if (!DefRegs[Reg])
+        if (!DefRegs.contains(Reg))
           MIB.addReg(Reg, RegState::ImplicitDefine | RegState::Dead);
       }