swift234

SILCombiner: Collapse existential pack/unpack/ref_to_raw_pointer casts

Also collapse existential pack/unpack/unchecked_ref_cast.

 (ref_to_raw_pointer (open_existential_ref (init_existential_ref x)))
  ->
 (ref_to_raw_pointer x)

 (unchecked_ref_cast (open_existential_ref (init_existential_ref x)))
  ->
 (unchecked_ref_cast x)

radar://18806550

Swift SVN r23032