swift234

Fix two bugs related to the casts optimizations.

The protocol_lookup the testcase from Interpreter test-cases exposed two bugs, once I tried to compiler with with -O:
 - SILCloner was generating an open_existential_ref from an open_existential_metatype instruction during cloning even if the existential in question was not a class existential.
 - DynamicCasts was not considering the fact that subclasses of a given class may implement a protocol, even if the class does not implement it.

Swift SVN r27260