[NewOptimizer] Domsort basic blocks

[Keno]

This sorts basic blocks into dominance order right after SSA construction.
Doing so has the benefit that linear traversals of the function
automatically become reverse post order traversals, or in other words,
we're guaranteed that during a linear traversal, we see any SSA
definitions before uses of those SSA values. This property is a prerequisite
for the incremental compactor to work correctly without back tracking.
It should also allow for simplifying codegen later, though not until
the new IR is the only thing supported by codegen.

Fixes core/asyncmap tests with the new optimizer enabled. I believe
those were the last tests that caused invalid IR generation (there's
still a few test failures due to the disabled alloc elim pass).

[Keno]

I have more work to put in on top of this, so merging this.