[SimpifyCFG] Remove recursion from FoldCondBranchOnPHI. NFCI.

[critson]

Summary

Avoid stack overflow errors on systems with small stack sizes
by removing recursion in FoldCondBranchOnPHI.

This is a simple change as the recursion was only iteratively
calling the function again on the same arguments.
Ideally this would be compiled to a tail call, but there is
no guarantee.

[lebedev.ri]

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

2727

Presumably this should return None?

2735–2741

This might be more readable, we don't really care how much recursion we have..

[critson]

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

2727

Yes, thanks!
I had forgotten what the NoneType was called.

2735–2741

Agreed.

[critson]

• Incorporate reviewer feedback

[lebedev.ri]

lg

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

static bool BlockIsSimpleEnoughToThreadThrough(BasicBlock *BB) {
   }
 
   return true;
 }
 
 /// If we have a conditional branch on a PHI node value that is defined in the
 /// same block as the branch and if any PHI entries are constants, thread edges
 /// corresponding to that entry to be branches to their ultimate destination.
-static bool FoldCondBranchOnPHI(BranchInst *BI, DomTreeUpdater *DTU,
-                                const DataLayout &DL, AssumptionCache *AC) {
+static Optional<bool> FoldCondBranchOnPHIImpl(BranchInst *BI,
+                                              DomTreeUpdater *DTU,
+                                              const DataLayout &DL,
+                                              AssumptionCache *AC) {
   BasicBlock *BB = BI->getParent();
   PHINode *PN = dyn_cast<PHINode>(BI->getCondition());
   // NOTE: we currently cannot transform this case if the PHI node is used
   // outside of the block.
   if (!PN || PN->getParent() != BB || !PN->hasOneUse())
     return false;
 
   // Degenerate case of a single entry PHI.
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
 
     if (DTU) {
       Updates.push_back({DominatorTree::Insert, PredBB, EdgeBB});
       Updates.push_back({DominatorTree::Delete, PredBB, BB});
 
       DTU->applyUpdates(Updates);
     }
 
-    // Recurse, simplifying any other constants.
-    return FoldCondBranchOnPHI(BI, DTU, DL, AC) || true;
+    // Signal repeat, simplifying any other constants.
+    return None;
   }
 
   return false;
 }
 
+static bool FoldCondBranchOnPHI(BranchInst *BI, DomTreeUpdater *DTU,
+                                const DataLayout &DL, AssumptionCache *AC) {
+  Optional<bool> Result;
+  bool EverChanged = false;
+  do {
+    // Note that None means "we changed things, but recurse further."
+    Result = FoldCondBranchOnPHIImpl(BI, DTU, DL, AC);
+    EverChanged |= Result == None || *Result;
+  } while (Result == None);
+  return EverChanged;
+}
+
 /// Given a BB that starts with the specified two-entry PHI node,
 /// see if we can eliminate it.
 static bool FoldTwoEntryPHINode(PHINode *PN, const TargetTransformInfo &TTI,
                                 DomTreeUpdater *DTU, const DataLayout &DL) {
   // Ok, this is a two entry PHI node.  Check to see if this is a simple "if
   // statement", which has a very simple dominance structure.  Basically, we
   // are trying to find the condition that is being branched on, which
   // subsequently causes this merge to happen.  We really want control