Some changes I was working on uncovered a latent bug where we would
emit a class_method instruction to call an allocating initializer
that did not have a vtable entry.
Previously this wasn't caught because the only example of this in
our test suite was in test/SILGen/objc_bridging_any.swift, which
did not test with IRGen; if it did, an IRGen crash would have been
observed.
Factor out some code duplication to prevent this from happening
again, and add a SILGen test that we emit a vtable entry in this
case, and that the test case passes IRGen also.