Rollup merge of #118929 - Zalathar:look-hir, r=cjgillot

coverage: Tidy up early parts of the instrumentor pass

This is extracted from #118237, which needed to be manually rebased anyway.

Unlike that PR, this one only affects the coverage instrumentor, and doesn't attempt to move any code into the MIR builder. That can be left to a future version of #118305, which can still benefit from these improvements.

So this is now mostly a refactoring of some internal parts of the instrumentor.