swift234

[enum-simplification] Make enum simplification global and teach it how to infer enum case from switch_enum.

With this change we now simplify 2192 retain, release operations on
enums an increase of ~56% over the previous stat, 1403 retain, release
operations.

The next step is to implement switch_enum simplification and pushing
releases on enums into switch regions (the real purpose of this pass).

rdar://17139960

Swift SVN r18740