[CodeGen] IndirectBrExpandPass: preserve Dominator Tree, if available

This fully de-pessimizes the common case of no indirectbr's,
(where we don't actually need to do anything to preserve domtree)
and avoids domtree recomputation in the case there were indirectbr's.

Note that two indirectbr's could have a common successor, and not all
successors of an indirectbr's are meant to survive the expansion.

Though, the code assumes that an indirectbr's doesn't have
duplicate successors, those *should* have been deduplicated
by simplifycfg or something already.