coverage: Simplify the injection of coverage statements

[Zalathar]

This is a follow-up to #116046 that I left out of that PR because I didn't want to make it any larger.

After the various changes we've made to how coverage data is stored and transferred, the old code structure for injecting coverage statements into MIR is built around a lot of constraints that don't exist any more. We can simplify it by replacing it with a handful of loops over the BCB node/edge counters and the BCB spans.

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@rustbot label +A-code-coverage

[rustbot]

r? @cjgillot

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[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[cjgillot]

Thanks.
One nit and r=me

[cjgillot]

Suggested change

	) -> impl Iterator<Item = ((BasicCoverageBlock, BasicCoverageBlock), &BcbCounter)> {
	) -> impl Iterator<Item = (BasicCoverageBlock, BasicCoverageBlock, &BcbCounter)> {

?
Should we add have a little doc-comment explaining which is which among the 2 bcbs?

[Zalathar]

The intent here is that it's a stream of key/value pairs, where each key is a directed edge in the BCB graph (represented by a pair of nodes). That's why I used a nested tuple, and didn't think it necessary to describe the order.

But it doesn't make much difference either way, so I can certainly change it.

[Zalathar]

Rebased, then flattened and documented bcb_edge_counters as requested (diff).

EDIT: And rebased again for good measure.

[cjgillot]

Thanks!
@bors r+

[bors]

📌 Commit ff02d92 has been approved by cjgillot

It is now in the queue for this repository.