[MachineCSE] Rewrite a loop checking if a block is in a set of blocks without using a set. NFC.

[mzolotukhin]

Summary

Using a set is unnecessary here an in some cases (see e.g. PR37277)
takes significant amount of time to just insert values into it. In this
particular case all we need is just to check if we find the block we are
looking for or not.

[efriedma]

LGTM

llvm/trunk/lib/CodeGen/MachineCSE.cpp

if (!HasVRegUse) {
     }
     if (!HasNonCopyUse)
       return false;
   }
 
   // Heuristics #3: If the common subexpression is used by PHIs, do not reuse
   // it unless the defined value is already used in the BB of the new use.
   bool HasPHI = false;
-  SmallPtrSet<MachineBasicBlock*, 4> CSBBs;
-  for (MachineInstr &MI : MRI->use_nodbg_instructions(CSReg)) {
-    HasPHI |= MI.isPHI();
-    CSBBs.insert(MI.getParent());
+  for (MachineInstr &UseMI : MRI->use_nodbg_instructions(CSReg)) {
+    HasPHI |= UseMI.isPHI();
+    if (UseMI.getParent() == MI->getParent())
+      return true;
   }
 
-  if (!HasPHI)
-    return true;
-  return CSBBs.count(MI->getParent());
+  return !HasPHI;
 }
 
 void MachineCSE::EnterScope(MachineBasicBlock *MBB) {
   DEBUG(dbgs() << "Entering: " << MBB->getName() << '\n');
   ScopeType *Scope = new ScopeType(VNT);
   ScopeMap[MBB] = Scope;
 }