Rollup merge of #122860 - Zalathar:unused, r=cjgillot

coverage: Re-enable `UnreachablePropagation` for coverage builds

This is a sequence of 3 related changes:
- Clean up the existing code that scans for unused functions
- Detect functions that were instrumented for coverage, but have had all their coverage statements removed by later MIR transforms (e.g. `UnreachablePropagation`)
- Re-enable `UnreachablePropagation` in coverage builds

Because we now detect functions that have lost their coverage statements, and treat them as unused, we don't need to worry about `UnreachablePropagation` removing all of those statements. This is demonstrated by `tests/coverage/unreachable.rs`.

Fixes #116171.

compiler/rustc_mir_transform/src/unreachable_prop.rs

@@ -14,11 +14,7 @@ pub struct UnreachablePropagation;
 impl MirPass<'_> for UnreachablePropagation {
     fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
         // Enable only under -Zmir-opt-level=2 as this can make programs less debuggable.
-
-        // FIXME(#116171) Coverage gets confused by MIR passes that can remove all
-        // coverage statements from an instrumented function. This pass can be
-        // re-enabled when coverage codegen is robust against that happening.
-        sess.mir_opt_level() >= 2 && !sess.instrument_coverage()
+        sess.mir_opt_level() >= 2
     }
 
     fn run_pass<'tcx>(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {