swift234

[sil-devirtualizer] Fix some bugs in the devirtualizer.

- Don't crash if a class_method instruction could not be devirtualized.
- Improve devirtualization of methods with generic parameters and using dependent types.
- Fix a bug in isBindableToSuperclassOf, uncovered while fixing the original bug reported in SR-1206.
  This bug could lead in certain cases to invocations of a wrong method from the base class, instead
  of using a method from a derived class.

rdar://25891588   and SR-1206