swift234

switch the frontend to generate swift_retain_noresult calls instead of
swift_retain calls.  The pertinent difference is that the former can be
marked nocapture, allowing general LLVM optimizations more flexibility.

With this change, early-cse is able to zap 9 more instructions, and 3
more functions are able to be marked nocapture by functionattrs in the
stdlib.


Swift SVN r2043