[PlaceSafepoints] Assert that the gc.safepoint_poll function is present in the module.

If running the PlaceSafepoints pass on a module which doesn't have the
gc.safepoint_poll function without disabling entry and backedge safepoints,
previously the pass crashed with an obscure error because of a null pointer.
Now it fails the assert instead.

llvm-svn: 256580

llvm/lib/Transforms/Scalar/PlaceSafepoints.cpp

@@ -762,6 +762,7 @@ InsertSafepointPoll(Instruction *InsertBefore,
   // path call - where we need to insert a safepoint (parsepoint).
 
   auto *F = M->getFunction(GCSafepointPollName);
+  assert(F && "gc.safepoint_poll function is missing");
   assert(F->getType()->getElementType() ==
          FunctionType::get(Type::getVoidTy(M->getContext()), false) &&
          "gc.safepoint_poll declared with wrong type");