swift234

SimplifyCFG: simplify the switch_enum -> select_enum conversion.

The main intention for this change is to eliminate the use of the post/dominator trees in this transformation.
These were re-calculated on every conversion which caused long compile times for functions with lot of switch_enum instructions: rdar://problem/24410167

Beside that, the code for collecting the target-block's predecessors is now simpler. It's not necessary to handle arbitrary control flow pathes because jump threading is simplifying the CFG anyway.

Now SimplifyCFG does not use the PostDominanceAnalysis anymore.