[NFC] Use assert instead of checking the guaranteed condition

From preconditions it is known that either A dominates B or
B dominates A. If A does not dominate B, we do not really need
to check it. Assert should be enough. Should save some compile
time.

Author: xortator

llvm/lib/Transforms/Scalar/IndVarSimplify.cpp

@@ -2402,15 +2402,18 @@ bool IndVarSimplify::optimizeLoopExits(Loop *L, SCEVExpander &Rewriter) {
   // Visit our exit blocks in order of dominance. We know from the fact that
   // all exits must dominate the latch, so there is a total dominance order
   // between them.
-  llvm::sort(ExitingBlocks,
-             [&](BasicBlock *A, BasicBlock *B) {
+  llvm::sort(ExitingBlocks, [&](BasicBlock *A, BasicBlock *B) {
                // std::sort sorts in ascending order, so we want the inverse of
                // the normal dominance relation.
                if (A == B) return false;
-               if (DT->properlyDominates(A, B)) return true;
-               if (DT->properlyDominates(B, A)) return false;
-               llvm_unreachable("expected total dominance order!");
-             });
+               if (DT->properlyDominates(A, B))
+                 return true;
+               else {
+                 assert(DT->properlyDominates(B, A) &&
+                        "expected total dominance order!");
+                 return false;
+               }
+  });
 #ifdef ASSERT
   for (unsigned i = 1; i < ExitingBlocks.size(); i++) {
     assert(DT->dominates(ExitingBlocks[i-1], ExitingBlocks[i]));