[SimplifyCFG] Cleanup redundant conditions [NFC].

[asbirlea]

Summary

Since the for loop iterates over BB's predecessors, the branch conditions found must have BB as one of the successors.
For an unconditional branch the successor must be BB, added assert.
For a conditional branch, one of the two successors must be BB, simplify else if to else and assert.
Sink common instructions outside the if/else block.

[sanjoy.google]

lgtm

lib/Transforms/Utils/SimplifyCFG.cpp

if (&BB->front() != UI)
     return Changed;
 
   SmallVector<BasicBlock *, 8> Preds(pred_begin(BB), pred_end(BB));
   for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
     Instruction *TI = Preds[i]->getTerminator();
     IRBuilder<> Builder(TI);
     if (auto *BI = dyn_cast<BranchInst>(TI)) {
       if (BI->isUnconditional()) {
-        if (BI->getSuccessor(0) == BB) {
-          new UnreachableInst(TI->getContext(), TI);
-          TI->eraseFromParent();
-          Changed = true;
-        }
+        assert(BI->getSuccessor(0) == BB && "Incorrect CFG");
       } else {
         Value* Cond = BI->getCondition();
         if (BI->getSuccessor(0) == BB) {
           Builder.CreateAssumption(Builder.CreateNot(Cond));
           Builder.CreateBr(BI->getSuccessor(1));
-          EraseTerminatorAndDCECond(BI);
-          Changed = true;
-        } else if (BI->getSuccessor(1) == BB) {
+        } else {
+          assert(BI->getSuccessor(1) == BB && "Incorrect CFG");
           Builder.CreateAssumption(Cond);
           Builder.CreateBr(BI->getSuccessor(0));
-          EraseTerminatorAndDCECond(BI);
-          Changed = true;
-        }
-      }
+        }
     } else if (auto *SI = dyn_cast<SwitchInst>(TI)) {
       SwitchInstProfUpdateWrapper SU(*SI);
       for (auto i = SU->case_begin(), e = SU->case_end(); i != e;) {
         if (i->getCaseSuccessor() != BB) {
           ++i;
           continue;
         }
         BB->removePredecessor(SU->getParent());