Fix segfault in FlattenCFG

[dfukalov]

Summary

Got segfault on dereferencing nullptr since getTerminator() returned not a BranchInst but a SwitchInst.
Was found in our OpenCL compiler with HSAIL BE, was not able to create regression test for vanilla llvm (
Fixed other similar places in the file.

[jvesely]

adding Matt to cc.
the change looks OK to me, although I've never worked with flattencfg.

If the segfault was generated by legal LLVM IR, it should be easy to just add it to 'test/Transforms/Util/flattencfg.ll'

[arsenm]

Running the standalone pass should be easy to add tests for this

lib/Transforms/Utils/FlattenCFG.cpp

if (!PBI2 || !PBI2->isUnconditional() ||
       return false;
 
     // Do branch inversion.
     BasicBlock *CurrBlock = LastCondBlock;
     bool EverChanged = false;
     for (;CurrBlock != FirstCondBlock;
           CurrBlock = CurrBlock->getSinglePredecessor()) {
       BranchInst *BI = dyn_cast<BranchInst>(CurrBlock->getTerminator());
+      if (!BI)
+        continue;
       CmpInst *CI = dyn_cast<CmpInst>(BI->getCondition());
       if (!CI)
         continue;
 
       CmpInst::Predicate Predicate = CI->getPredicate();
       // Canonicalize icmp_ne -> icmp_eq, fcmp_one -> fcmp_oeq
       if ((Predicate == CmpInst::ICMP_NE) || (Predicate == CmpInst::FCMP_ONE)) {
         CI->setPredicate(ICmpInst::getInversePredicate(Predicate));
bool FlattenCFGOpt::FlattenParallelAndOr(BasicBlock *BB, IRBuilder<> &Builder) {
   // PS2 should not contain PHI node.
   PHI = dyn_cast<PHINode>(PS2->begin());
   if (PHI)
     return false;
 
   // Do the transformation.
   BasicBlock *CB;
   BranchInst *PBI = dyn_cast<BranchInst>(FirstCondBlock->getTerminator());
+  if (!PBI)
+    return false;
   bool Iteration = true;
   IRBuilder<>::InsertPointGuard Guard(Builder);
   Value *PC = PBI->getCondition();
 
   do {
     CB = PBI->getSuccessor(1 - Idx);
     // Delete the conditional branch.
     FirstCondBlock->getInstList().pop_back();
if (isa<PHINode>(CI) || CI->mayHaveSideEffects() ||
       return false;
   }
 
   // Merge \param SecondEntryBlock into \param FirstEntryBlock.
   FirstEntryBlock->getInstList().pop_back();
   FirstEntryBlock->getInstList()
       .splice(FirstEntryBlock->end(), SecondEntryBlock->getInstList());
   BranchInst *PBI = dyn_cast<BranchInst>(FirstEntryBlock->getTerminator());
+  if (!PBI)
+    return false;
   Value *CC = PBI->getCondition();
   BasicBlock *SaveInsertBB = Builder.GetInsertBlock();
   BasicBlock::iterator SaveInsertPt = Builder.GetInsertPoint();
   Builder.SetInsertPoint(PBI);
   Value *NC = Builder.CreateOr(CInst1, CC);
   PBI->replaceUsesOfWith(CC, NC);
   Builder.SetInsertPoint(SaveInsertBB, SaveInsertPt);