swift234

[inst-simplify] (upcast X2->X (ref_to_objectPointer X->ObjectPtr (objectpointer_to-ref ObjectPtr->X2 alpha))) -> alpha

Since we don't have a downcast instruction, a common downcast pattern
round trip is:

  upcast : $X2 -> $X
  ref_to_object_pointer : $X -> $Builtin.ObjectPointer
  object_pointer_to_ref : $Builtin.ObjectPointer -> $X2

We can RAUW the object pointer with the operand of the upcast. This
canonicalization helps out mem2reg and devirtualization as well.

Swift SVN r16515