Skip MIR pass `UnreachablePropagation` when coverage is enabled
When coverage instrumentation and MIR opts are both enabled, coverage relies on two assumptions:
- MIR opts that would delete `StatementKind::Coverage` statements instead move them into bb0 and change them to `CoverageKind::Unreachable`.
- MIR opts won't delete all `CoverageKind::Counter` statements from an instrumented function.
Most MIR opts naturally satisfy the second assumption, because they won't remove coverage statements from bb0, but `UnreachablePropagation` can do so if it finds that bb0 is unreachable. If this happens, LLVM thinks the function isn't instrumented, and it vanishes from coverage reports.
A proper solution won't be possible until after per-function coverage info lands in #116046, but for now we can avoid the problem by turning off this particular pass when coverage instrumentation is enabled.
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cc `@cjgillot` since I found this while investigating coverage problems encountered by #113970
`@rustbot` label +A-code-coverage +A-mir-opt
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