Better dominators
This commit replaces the naive algorithm for replacing dominator
trees by a faster implementation based on the Semi-NCA algorithm
(reference in the code comments). LLVM recently switched to this
algorithm and found it to be faster in practice than SLT (which
it used before). It is also slightly easier to implement. More
importantly though, it should easily extend to dynamic dominators.
This fixes the preformance problems in dominator construction noted
in #25927 and should provide a basis for a dynamic dominator
implementation to fix #29107.
