Rollup merge of #116654 - Zalathar:reloop-traversal, r=oli-obk

coverage: Clarify loop-edge detection and graph traversal

This is a collection of improvements to two semi-related pieces of code:

- The code in `counters` that detects which graph edges don't exit a loop, and would therefore be good candidates to have their coverage computed as an expression rather than having a physical counter.
- The code in `graph` that traverses the coverage BCB graph in a particular order, and tracks loops and loop edges along the way (which is relevant to the above).

I was originally only planning to make the `graph` changes, but there was going to be a lot of indentation churn in `counters` anyway, and once I started looking I noticed a lot of opportunities for simplification.

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`@rustbot` label +A-code-coverage

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