swift234

SimplifyCFG: Fix dominatorBaseSimplify

Separate it into two phases: one that simplifies based on the domtree and one
that changes the CFG. Recompute the domtree in between.

Don't process unreachable blocks created by checked cast jump threading. The
dominator tree update in checked cast jump threading is buggy - recalclate the
dominator tree for now.

I measured no changes in performance and a +0.1-0.2% change in compile time of
'ninja -j1 swift-stdlib'.

rdar://20404463

Swift SVN r26979