[LV] Split off memcheck block really at the first check

[anemet]

Summary

I've noticed this while trying to move addRuntimeCheck to LoopAccessAnalysis.

I think that the intention was to early exit from the overflow checking before
the code for the memchecks. This is the entire reason why we compute
FirstCheckInst but then we don't use that as the splitting instruction but the
final check. Looks like an oversight.

[aschwaighofer]

LGTM

[anemet]

r228056

lib/Transforms/Vectorize/LoopVectorize.cpp

void InnerLoopVectorizer::createEmptyLoop() {
   // checks into a separate block to make the more common case of few elements
   // faster.
   Instruction *MemRuntimeCheck;
   std::tie(FirstCheckInst, MemRuntimeCheck) =
       addRuntimeCheck(LastBypassBlock->getTerminator());
   if (MemRuntimeCheck) {
     // Create a new block containing the memory check.
     BasicBlock *CheckBlock =
-        LastBypassBlock->splitBasicBlock(MemRuntimeCheck, "vector.memcheck");
+        LastBypassBlock->splitBasicBlock(FirstCheckInst, "vector.memcheck");
     if (ParentLoop)
       ParentLoop->addBasicBlockToLoop(CheckBlock, *LI);
     LoopBypassBlocks.push_back(CheckBlock);
 
     // Replace the branch into the memory check block with a conditional branch
     // for the "few elements case".
     Instruction *OldTerm = LastBypassBlock->getTerminator();
     BranchInst::Create(MiddleBlock, CheckBlock, Cmp, OldTerm);