coverage: Sort the expansion tree to help choose a single BCB for child expansions

[Zalathar]

This PR is another incremental step on the road towards proper coverage instrumentation of expansion regions.

When creating coverage mappings for each relevant span in a function body, the current implementation also needs to do a separate tree traversal for each child expansion (e.g. macro calls like println!("foo")), in order to associate a single control-flow point (BCB) with that macro call's span.

This PR changes things so that we now keep track of the “minimum” and ”maximum” BCB associated with each node in the expansion tree, which makes it much easier for the parent node to get a single BCB (min or max) for each of its child expansions.

In order to make this (relatively) easy, we first need to sort the tree nodes into depth-first order. Once that's taken care of, we can iterate over all the nodes in reverse order, knowing that each node's children will have been visited before visiting the node itself.

[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[rustbot]

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