[cast-optimizer] Fix incorrect handling of returned pointers.
Put simply, the method Type::getAnyNominal() returns either a nominal type or a
nullptr. We are then casting it using dyn_cast to a ProtocolDecl. dyn_cast is
not allowed to handle nullptrs... hence the crash. The fix is to use the API
dyn_cast_or_null, the specific cast meant for this case!
In terms of how this effects the flow of the code in such a case, we will just
bail early if we have a non-nominal type today (which to me means at least
metatypes, which is this case).
I included a test case that we crash on without my change today and just do not
optimize.
rdar://87989767
