intel/compiler: Move dominance tree data structure into idom_tree object

intel/compiler: Move dominance tree data structure into idom_tree object

It makes sense to keep the result of analysis passes independent from
the IR itself.  Instead of representing the idom tree as a pointer in
each basic block pointing to its immediate dominator, the whole
dominator tree is represented separately from the IR as an array of
pointers inside the idom_tree object.  This has the advantage that
it's no longer possible to use stale dominance results by accident
without having called require() beforehand, which makes sure that the
idom tree is recalculated if necessary.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4012>