swift234

For Swift-rooted class hierarchies, emit self_downcast as a bitcast.

Swift-rooted class hierarchies don't permit the replacement of "self",
so it's guaranteed that the 'self' returned from a 'super.init'
invocation is identical to the previous self. Elide the unconditional
checked downcast (that manifests as a call to
swift_dynamicCastClassUnconditional in the runtime) by simply putting
a bitcast in its place.

For Objective-C-rooted hierarchies, 'self' can be replaced so memory
safety dictates that we keep the
swift_dynamicCastClassUnconditional(). Note that this path is still
unsafe against nil returns.

Swift SVN r12384