Tell downward splitting from the previous upward splitting.
- By upward splitting, a new basic block (NewBB) is split and inherits part or all predecessors of the original basic block (OrigBB) and has OrigBB as its only successor.
- By downward splitting, NewBB is split and inherits part of all successors of OrigBB and has OrigBB as its only predecessor.
So far, we only support upward splitting for (forward) dominator tree. For post-dominator tree, a upward splitting in the forward graph is in fact a downward splitting in the backward graph. This renders the current splitting support invalid if we want to incrementally update post-dominator tree.
Minor name change on the interface checking whether a dominator tree node is reachable from entry to use it for both dominator and post-dominator trees.