Auto merge of #117123 - Zalathar:bad-counter-ids, r=petrochenkov

coverage: Consistently remove unused counter IDs from expressions/mappings

If some coverage counters were removed by MIR optimizations, we need to take care not to refer to those counter IDs in coverage mappings, and instead replace them with a constant zero value. If we don't, `llvm-cov` might see a too-large counter ID and silently discard the entire function from its coverage reports.

Fixes #117012.

Author: bors