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

[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@256580 91177308-0d34-0410-b5e6-96231b3b80d8