[ARM] Use DenseSet instead of DenseMap. NFC (#131978)

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 already 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);
       }