[NFC][SimplifyCFG] FoldBranchToCommonDest(): somewhat better structure weight updating code

Hoist the successor updating out of the code that deals with branch
weight updating, and hoist the 'has weights' check from the latter,
making code more consistent and easier to follow.

Author: LebedevRI

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

@@ -3005,13 +3005,11 @@ bool llvm::FoldBranchToCommonDest(BranchInst *BI, DomTreeUpdater *DTU,
     PBI->setCondition(NewCond);
 
     uint64_t PredTrueWeight, PredFalseWeight, SuccTrueWeight, SuccFalseWeight;
-    bool HasWeights =
-        extractPredSuccWeights(PBI, BI, PredTrueWeight, PredFalseWeight,
-                               SuccTrueWeight, SuccFalseWeight);
-    SmallVector<uint64_t, 8> NewWeights;
+    if (extractPredSuccWeights(PBI, BI, PredTrueWeight, PredFalseWeight,
+                               SuccTrueWeight, SuccFalseWeight)) {
+      SmallVector<uint64_t, 8> NewWeights;
 
-    if (PBI->getSuccessor(0) == BB) {
-      if (HasWeights) {
+      if (PBI->getSuccessor(0) == BB) {
         // PBI: br i1 %x, BB, FalseDest
         // BI:  br i1 %y, UniqueSucc, FalseDest
         // TrueWeight is TrueWeight for PBI * TrueWeight for BI.
@@ -3023,11 +3021,7 @@ bool llvm::FoldBranchToCommonDest(BranchInst *BI, DomTreeUpdater *DTU,
         NewWeights.push_back(PredFalseWeight *
                                  (SuccFalseWeight + SuccTrueWeight) +
                              PredTrueWeight * SuccFalseWeight);
-      }
-      PBI->setSuccessor(0, UniqueSucc);
-    }
-    if (PBI->getSuccessor(1) == BB) {
-      if (HasWeights) {
+      } else {
         // PBI: br i1 %x, TrueDest, BB
         // BI:  br i1 %y, TrueDest, UniqueSucc
         // TrueWeight is TrueWeight for PBI * TotalWeight for BI +
@@ -3038,17 +3032,20 @@ bool llvm::FoldBranchToCommonDest(BranchInst *BI, DomTreeUpdater *DTU,
         // FalseWeight is FalseWeight for PBI * FalseWeight for BI.
         NewWeights.push_back(PredFalseWeight * SuccFalseWeight);
       }
-      PBI->setSuccessor(1, UniqueSucc);
-    }
-    if (NewWeights.size() == 2) {
+
       // Halve the weights if any of them cannot fit in an uint32_t
       FitWeights(NewWeights);
 
       SmallVector<uint32_t, 8> MDWeights(NewWeights.begin(), NewWeights.end());
       setBranchWeights(PBI, MDWeights[0], MDWeights[1]);
+
+      // TODO: If BB is reachable from all paths through PredBlock, then we
+      // could replace PBI's branch probabilities with BI's.
     } else
       PBI->setMetadata(LLVMContext::MD_prof, nullptr);
 
+    PBI->setSuccessor(PBI->getSuccessor(0) != BB, UniqueSucc);
+
     if (DTU)
       DTU->applyUpdates({{DominatorTree::Insert, PredBlock, UniqueSucc},
                          {DominatorTree::Delete, PredBlock, BB}});
@@ -3058,9 +3055,6 @@ bool llvm::FoldBranchToCommonDest(BranchInst *BI, DomTreeUpdater *DTU,
     if (MDNode *LoopMD = BI->getMetadata(LLVMContext::MD_loop))
       PBI->setMetadata(LLVMContext::MD_loop, LoopMD);
 
-    // TODO: If BB is reachable from all paths through PredBlock, then we
-    // could replace PBI's branch probabilities with BI's.
-
     // Copy any debug value intrinsics into the end of PredBlock.
     for (Instruction &I : *BB) {
       if (isa<DbgInfoIntrinsic>(I)) {