Don't abuse reinterpret cast to do something the API of PointerUnion

provides proper support for. This was caught by
-Wundefined-reinterpret-cast, and I think a reasonable case for it to
warn on.

Also use is<...> instead of dyn_cast<...> when the result isn't needed.

This whole thing should probably switch to using UsuallyTinyPtrVector.

llvm-svn: 130707