[JumpThread] Simplify CmpInst-as-Condition branch-folding a bit.

[trentxintong]

Summary

Simplify CmpInst-as-Condition branch-folding a bit.

[trentxintong]

Gentle ping.

[efriedma]

LGTM.

lib/Transforms/Scalar/JumpThreading.cpp

bool JumpThreadingPass::ProcessBlock(BasicBlock *BB) {
 
   if (CmpInst *CondCmp = dyn_cast<CmpInst>(CondInst)) {
     // If we're branching on a conditional, LVI might be able to determine
     // it's value at the branch instruction.  We only handle comparisons
     // against a constant at this time.
     // TODO: This should be extended to handle switches as well.
     BranchInst *CondBr = dyn_cast<BranchInst>(BB->getTerminator());
     Constant *CondConst = dyn_cast<Constant>(CondCmp->getOperand(1));
-    if (CondBr && CondConst && CondBr->isConditional()) {
+    if (CondBr && CondConst) {
+      // We should have returned as soon as we turn a conditional branch to
+      // unconditional. Because its no longer interesting as far as jump
+      // threading is concerned.
+      assert(CondBr->isConditional() && "Threading on unconditional terminator");
+
       LazyValueInfo::Tristate Ret =
         LVI->getPredicateAt(CondCmp->getPredicate(), CondCmp->getOperand(0),
                             CondConst, CondBr);
       if (Ret != LazyValueInfo::Unknown) {
         unsigned ToRemove = Ret == LazyValueInfo::True ? 1 : 0;
         unsigned ToKeep = Ret == LazyValueInfo::True ? 0 : 1;
         CondBr->getSuccessor(ToRemove)->removePredecessor(BB, true);
         BranchInst::Create(CondBr->getSuccessor(ToKeep), CondBr);
         CondBr->eraseFromParent();
         if (CondCmp->use_empty())
           CondCmp->eraseFromParent();
         else if (CondCmp->getParent() == BB) {
           // If the fact we just learned is true for all uses of the
           // condition, replace it with a constant value
           auto *CI = Ret == LazyValueInfo::True ?
             ConstantInt::getTrue(CondCmp->getType()) :
             ConstantInt::getFalse(CondCmp->getType());
           CondCmp->replaceAllUsesWith(CI);
           CondCmp->eraseFromParent();
         }
         return true;
       }
-    }
 
-    if (CondBr && CondConst && TryToUnfoldSelect(CondCmp, BB))
+      // We did not manage to simplify this branch, try to see whether
+      // CondCmp depends on a known phi-select pattern.
+      if (TryToUnfoldSelect(CondCmp, BB))
         return true;
     }
+  }
 
   // Check for some cases that are worth simplifying.  Right now we want to look
   // for loads that are used by a switch or by the condition for the branch.  If
   // we see one, check to see if it's partially redundant.  If so, insert a PHI
   // which can then be used to thread the values.
   //
   Value *SimplifyValue = CondInst;
   if (CmpInst *CondCmp = dyn_cast<CmpInst>(SimplifyValue))