AMDGPU: change a redundant if () to an assert(). NFC

[nhaehnle]

Summary

I've been carrying this change around with me for a while, because the if ()
managed to confuse me while following the code. All callers ensure that the
assertion holds.

[• tstellarAMD]

LGTM.

lib/Target/AMDGPU/SIAnnotateControlFlow.cpp

void SIAnnotateControlFlow::handleLoop(BranchInst *Term) {
   }
 
   Term->setCondition(CallInst::Create(Loop, Arg, "", Term));
   push(Term->getSuccessor(0), Arg);
 }/// \brief Close the last opened control flow
 void SIAnnotateControlFlow::closeControlFlow(BasicBlock *BB) {
   llvm::Loop *L = LI->getLoopFor(BB);
 
-  if (Stack.back().first != BB)
-    return;
+  assert(Stack.back().first == BB);
 
   if (L && L->getHeader() == BB) {
     // We can't insert an EndCF call into a loop header, because it will
     // get executed on every iteration of the loop, when it should be
     // executed only once before the loop.
     SmallVector <BasicBlock*, 8> Latches;
     L->getLoopLatches(Latches);