swift234

SIL: Turn conditional checked casts into a branch instruction.

Replace the existing suite of checked cast instructions with:

- unconditional_checked_cast, which performs an unconditional cast that aborts on failure (like the former downcast unconditional); and
- checked_cast_br, which performs a conditional pass and branches on whether the cast succeeds, passing the result to the true branch as an argument.

Both instructions take a CheckedCastKind that discriminates the different casting modes formerly discriminated by instruction type. This eliminates a source of null references in SIL and eliminates null SIL addresses completely.

Swift SVN r8696