swift234

[sil-combine] Rewrite unchecked_addr_cast to access layout compatible first element fields via typed GEPS.

A first element field of a nominal type is either the first element of a
struct or the first payload of an enum. We currently allow the stdlib to
rappel into struct heirarchies using reinterpretCast. This patch teaches
the optimizer how to rewrite such unchecked_addr_cast into
unchecked_enum_data_addr and struct_element_addr instructions. Then
Mem2Reg and Load Store Forwarding will remove the allocation generated
by such uses of reinterpret cast.

<rdar://problem/16703656>

Swift SVN r18977