swift234

Be more conservative about stripping upcasts in devirtualization.

This fixes two issues that I ran into with the devirtualizer and also
paves the path to more simplification of the devirtualizer code.

Both issues are due to having a very liberal definition of what can be
considered an upcast.

In one case, we tunnel through an unchecked_ref_cast and then fail to
devirtualize because the source type is not a class
type (rdar://problem/20115523). We would be better off sticking to the
class type we started with.

In the other case, we tunnel through an unchecked_ref_bit_cast and this
results in an attempt to insert a checked_cast_br between unrelated
types (rdar://problem/20117782).

Swift SVN r25976