swift234

Second part of the cast folding and type-casts re-factoring.

This patch does the following:
- Moves the logic for handling special-case of converting to/from AnyObject.Protocol (and existential.Protocol in general) into DynamicCasts, where all other cases are handled already.
- Moves the peephole which was folding checked_cast_br into an unchecked cast and branch from sil-combine into sil-simplify-cfg, because it is a better place for it, since this peephole affects the CFG. The corresponding test is also moved from sil_combine.sil into simplify_cfg.sil.
- Adds a few checked_cast_br peepholes to sil-combine.   They try to simplify checked_cond_br instructions using existential metatypes by propagating a concrete type whenever it can be determined statically.
- Adds a new test with a lot of test-cases that make sure we are really folding many type-checks at compile-time now.

Swift SVN r25504