swift234

Canonicalize 'release' instructions and make them amenable to code motion.

This is r31724 with the added restriction that the checked branch has to be
'exact'. At some point we need to look into casts that cast value_metatypes
like this:

  checked_cast_br %0 : $@thick C.Type to $AnyObject, bb1, bb2

Swift SVN r31734