[LLVM][Support/ADT] Add assert for isPresent to dyn_cast.

This change adds an assert to dyn_cast that the value passed-in is present. In the past, this relied on the isa_impl assertion (which still works in many cases) but which we can tighten up for a better QoI.

The PointerUnion change is because it seems like (based on the call sites) the semantics of the member dyn_cast are actually dyn_cast_if_present.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D133221