[NFC] Instruction: introduce replaceSuccessorWith() function, use it

[lebedev.ri]

Summary

There is Instruction::getNumSuccessors(), Instruction::getSuccessor()
and Instruction::setSuccessor(), but no function to replace every
specified BasicBlock* successor with some other specified BasicBlock*.
I've found one place where it should clearly be used.

[craig.topper]

include/llvm/IR/Instruction.h

670

replaceSuccessorWith?

[lebedev.ri]

Rename to replaceSuccessor().

[craig.topper]

lib/IR/Instruction.cpp

679

Capitalize as Idx?

681

Drop the temporary variable and the curly braces?

[lebedev.ri]

Address review notes.

[lebedev.ri]

Hm, really drop braces, too.

[craig.topper]

LGTM

include/llvm/IR/Instruction.h

public:
 
   /// Return the specified successor. This instruction must be a terminator.
   BasicBlock *getSuccessor(unsigned Idx) const;
 
   /// Update the specified successor to point at the provided block. This
   /// instruction must be a terminator.
   void setSuccessor(unsigned Idx, BasicBlock *BB);
 
+  /// Change specified successor OldBB to point at the provided block.
+  /// This instruction must be a terminator.
+  void changeSuccessor(BasicBlock *OldBB, BasicBlock *NewBB);
+
   /// Methods for support type inquiry through isa, cast, and dyn_cast:
   static bool classof(const Value *V) {
     return V->getValueID() >= Value::InstructionVal;
   }
 
   //----------------------------------------------------------------------
   // Exported enumerations.
   //

lib/IR/Instruction.cpp

case Instruction::OPC:                                                       \
     return static_cast<CLASS *>(this)->setSuccessor(idx, B);
 #include "llvm/IR/Instruction.def"
   default:
     break;
   }
   llvm_unreachable("not a terminator");
 }
 
+void Instruction::changeSuccessor(BasicBlock *OldBB, BasicBlock *NewBB) {
+  for (unsigned idx = 0, NumSuccessors = Instruction::getNumSuccessors();
+       idx != NumSuccessors; ++idx) {
+    BasicBlock *CurrSuccessor = getSuccessor(idx);
+    if (CurrSuccessor == OldBB)
+      setSuccessor(idx, NewBB);
+  }
+}
+
 Instruction *Instruction::cloneImpl() const {
   llvm_unreachable("Subclass of Instruction failed to implement cloneImpl");
 }
 
 void Instruction::swapProfMetadata() {
   MDNode *ProfileData = getMetadata(LLVMContext::MD_prof);
   if (!ProfileData || ProfileData->getNumOperands() != 3 ||
       !isa<MDString>(ProfileData->getOperand(0)))

lib/Transforms/Utils/LoopSimplify.cpp

static BasicBlock *insertUniqueBackedgeBlock(Loop *L, BasicBlock *Preheader,
   // it from the backedge and add it to BEBlock.
   unsigned LoopMDKind = BEBlock->getContext().getMDKindID("llvm.loop");
   MDNode *LoopMD = nullptr;
   for (unsigned i = 0, e = BackedgeBlocks.size(); i != e; ++i) {
     Instruction *TI = BackedgeBlocks[i]->getTerminator();
     if (!LoopMD)
       LoopMD = TI->getMetadata(LoopMDKind);
     TI->setMetadata(LoopMDKind, nullptr);
-    for (unsigned Op = 0, e = TI->getNumSuccessors(); Op != e; ++Op)
-      if (TI->getSuccessor(Op) == Header)
-        TI->setSuccessor(Op, BEBlock);
+    TI->changeSuccessor(Header, BEBlock);
   }
   BEBlock->getTerminator()->setMetadata(LoopMDKind, LoopMD);
 
   //===--- Update all analyses which we must preserve now -----------------===//
 
   // Update Loop Information - we know that this block is now in the current
   // loop and all parent loops.
   L->addBasicBlockToLoop(BEBlock, *LI);