swift234

Split arc-optimizer into two passes: arc-optimize and arc-expand.
This makes the two phases independently testable, but is also the
right thing to do: previously we'd form swift_retain early enough
that inlining would inline them from previously optimized callees
into callers, and this would block some mid-level optimizations
from doing nice things (because swift_retain isn't no-escape).

It's a small thing, but doing this allows us to eliminate a few 
more "and x, 9223372036854775807"'s from the stdlib.  We also
end up doing a lot less optimizations because we do them early
instead of only having the optimizations exposed after inlining
deeply.



Swift SVN r2114