[SimplifyCFG][NFC] Improve compile time for TryToSimplifyUncondBranchFromEmptyBlock optimization.

In some pathological cases this optimization can spend an unreasonable amount of
time populating the set for predecessors of the successor block. This change
sinks some of that initializing to the point where it's actually necessary so we
can take advantage of the existing early-exits.

rdar://137063034

Author: aemerson

llvm/lib/Transforms/Utils/Local.cpp

@@ -1020,10 +1020,11 @@ static void replaceUndefValuesInPhi(PHINode *PN,
 // Only when they shares a single common predecessor, return true.
 // Only handles cases when BB can't be merged while its predecessors can be
 // redirected.
+// \p SuccPredsOut may be partially populated with the predecessors of Succ.
 static bool
 CanRedirectPredsOfEmptyBBToSucc(BasicBlock *BB, BasicBlock *Succ,
                                 const SmallPtrSetImpl<BasicBlock *> &BBPreds,
-                                const SmallPtrSetImpl<BasicBlock *> &SuccPreds,
+                                SmallPtrSetImpl<BasicBlock *> &SuccPredsOut,
                                 BasicBlock *&CommonPred) {
 
   // There must be phis in BB, otherwise BB will be merged into Succ directly
@@ -1042,7 +1043,8 @@ CanRedirectPredsOfEmptyBBToSucc(BasicBlock *BB, BasicBlock *Succ,
 
   // Get the single common predecessor of both BB and Succ. Return false
   // when there are more than one common predecessors.
-  for (BasicBlock *SuccPred : SuccPreds) {
+  for (BasicBlock *SuccPred : predecessors(Succ)) {
+    SuccPredsOut.insert(SuccPred);
     if (BBPreds.count(SuccPred)) {
       if (CommonPred)
         return false;
@@ -1166,7 +1168,7 @@ bool llvm::TryToSimplifyUncondBranchFromEmptyBlock(BasicBlock *BB,
     return false;
 
   SmallPtrSet<BasicBlock *, 16> BBPreds(pred_begin(BB), pred_end(BB));
-  SmallPtrSet<BasicBlock *, 16> SuccPreds(pred_begin(Succ), pred_end(Succ));
+  SmallPtrSet<BasicBlock *, 16> SuccPreds;
 
   // The single common predecessor of BB and Succ when BB cannot be killed
   BasicBlock *CommonPred = nullptr;
@@ -1182,6 +1184,10 @@ bool llvm::TryToSimplifyUncondBranchFromEmptyBlock(BasicBlock *BB,
   if ((!BBKillable && !BBPhisMergeable) || introduceTooManyPhiEntries(BB, Succ))
     return false;
 
+  // SuccPreds may not have been fully filled by CanRedirectPredsOfEmptyBBToSucc
+  // so finish it here.
+  SuccPreds.insert(pred_begin(Succ), pred_end(Succ));
+
   // Check to see if merging these blocks/phis would cause conflicts for any of
   // the phi nodes in BB or Succ. If not, we can safely merge.