Rollup merge of #124615 - Zalathar:extracted-mappings, r=davidtwco

coverage: Further simplify extraction of mapping info from MIR

This is another round of rearrangement and simplification that builds on top of the changes made to mapping-extraction by #124603.

The overall theme is to take the computation of `bcb_has_mappings` and `test_vector_bitmap_bytes` out of the main body of `generate_coverage_spans`, which then lets us perform a few other small changes that had previously been held up by the need to work around those computations.

Author: matthiaskrgr