[SimplifyCFG] Sanity assert in iterativelySimplifyCFG

We observe a hang within iterativelySimplifyCFG due to infinite
loop execution. Currently, there is no limit to this loop, so
in case of bug it just works forever. This patch adds an assert
that will break it after 1000 iterations if it didn't converge.

llvm/lib/Transforms/Scalar/SimplifyCFGPass.cpp

@@ -224,7 +224,11 @@ static bool iterativelySimplifyCFG(Function &F, const TargetTransformInfo &TTI,
   SmallVector<WeakVH, 16> LoopHeaders(UniqueLoopHeaders.begin(),
                                       UniqueLoopHeaders.end());
 
+  unsigned IterCnt = 0;
+  (void)IterCnt;
   while (LocalChange) {
+    assert(IterCnt++ < 1000 &&
+           "Sanity: iterative simplification didn't converge!");
     LocalChange = false;
 
     // Loop over all of the basic blocks and remove them if they are unneeded.