[LV] Remove redundant basic block split

[gilr]

Summary

This patch is part of D28975's breakdown.

The process of generating the control flow guarding predicated instructions includes:
(1) a call to SplitBlock(), which splits the basic block after the predicated instruction, producing:

--> HEAD -->  PRED --> TAIL -->

(2) a call to SplitBblockAndInsertIfThen() which splits the basic block before the predicated instruction into a "triangle", resulting
in a redundant basic block with an unconditional branch:

--> HEAD -->  PRED --> MERGE --> TAIL -->
          +----------------------^

By removing (1) we get:

--> HEAD -->  PRED --> MERGE/TAIL -->
          +--------------------^

[mkuper]

Makes sense. LGTM.

llvm/trunk/lib/Transforms/Vectorize/LoopVectorize.cpp

void InnerLoopVectorizer::predicateInstructions() {
   //
   // pred.sdiv.continue55:
   //  %20 = phi <2 x i32> [ %14, %pred.sdiv.continue ], [ %19, %pred.sdiv.if54 ]
   //  %predphi = select <2 x i1> %8, <2 x i32> %20, <2 x i32> %5
 
   for (auto KV : PredicatedInstructions) {
     BasicBlock::iterator I(KV.first);
     BasicBlock *Head = I->getParent();
-    auto *BB = SplitBlock(Head, &*std::next(I), DT, LI);
     auto *T = SplitBlockAndInsertIfThen(KV.second, &*I, /*Unreachable=*/false,
                                         /*BranchWeights=*/nullptr, DT, LI);
     I->moveBefore(T);
     sinkScalarOperands(&*I);
 
-    I->getParent()->setName(Twine("pred.") + I->getOpcodeName() + ".if");
-    BB->setName(Twine("pred.") + I->getOpcodeName() + ".continue");
+    BasicBlock *PredicatedBlock = I->getParent();
+    Twine BBNamePrefix = Twine("pred.") + I->getOpcodeName();
+    PredicatedBlock->setName(BBNamePrefix + ".if");
+    PredicatedBlock->getSingleSuccessor()->setName(BBNamePrefix + ".continue");
 
     // If the instruction is non-void create a Phi node at reconvergence point.
     if (!I->getType()->isVoidTy()) {
       Value *IncomingTrue = nullptr;
       Value *IncomingFalse = nullptr;
 
       if (I->hasOneUse() && isa<InsertElementInst>(*I->user_begin())) {
         // If the predicated instruction is feeding an insert-element, move it