[LoopReroll] Don't crash on dead code

If a PHI has no users, don't crash; bail gracefully. This shouldn't
happen often, but we can make no guarantees that previous passes didn't leave
dead code around.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@229405 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Scalar/LoopRerollPass.cpp

@@ -511,6 +511,8 @@ void LoopReroll::SimpleLoopReduction::add(Loop *L) {
   // (including the PHI), except for the last value (which is used by the PHI
   // and also outside the loop).
   Instruction *C = Instructions.front();
+  if (C->user_empty())
+    return;
 
   do {
     C = cast<Instruction>(*C->user_begin());