Rollup merge of #131918 - Zalathar:counters, r=nnethercote

coverage: Make counter creation handle node/edge counters more uniformly

Similar to #130380, this is another round of small improvements informed by my ongoing attempts to overhaul coverage counter creation.

One of the big benefits is getting rid of the awkward special-case that would sometimes attach an edge counter to a node instead. That was needed by the code that chooses which out-edge should be given a counter expression, but we can avoid that by making the corresponding check a little smarter.

I've also renamed several things to be simpler and more consistent, which should help with future changes.