Previously, there were two implementations with identical behavior to
erase a node from a dominator tree, one in the DomTreeBase and one in
SemiNCAInfo. Remove the latter, as it is completely redundant.
Also, use getNode() instead of a direct access into DomTreeNodes. This
will simplify replacing the data structure of DomTreeNodes later on.
While at it, also use swap+pop_back instead of erase when removing a
node from the children vector to avoid O(n) copy. This slightly changes
the order of the tree nodes after removal, but should have no impact.
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