[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