swift234

Revert "Introduce the self_downcast SIL instruction to downcast the result of super.init."

We need to model the difference between Objective-C- and Swift-rooted
class hierarchies in SIL. IRGen is too late to handle nil returns.

This reverts commit 549db981ea0136a67aee3029aefe18a05d3c8833.

Swift SVN r12400