swift234

[sil-combine] ({struct_extract,unchecked_enum_data}  (unchecked_ref_bit_cast)) -> unchecked_ref_bit_cast when the two instructions  are just layout compatible bit casts.

Also added identity cast simplifications for unchecked_*_bit_cast to
SimplifyInstruction.

<rdar://problem/17965742>


Swift SVN r21164