swift234

StackPromotion: Ignore unreachable blocks in post-dominator tree.

Unreachable blocks prevented stack promotion in some cases.
Now we use our own post-dominator tree which ignores unreachable blocks instead of the standard post-dominator tree provided by the PostDominanceAnalysis.
Unreachable blocks (better: unreachable sub-graphs) are of no interrest because we don't have to insert the dealloc instructions in unreachable blocks anyway.