Auto merge of #123272 - saethlin:reachable-mono-cleanup, r=cjgillot

Only collect mono items from reachable blocks

Fixes the wrong comment pointed out in: rust-lang/rust#121421 (comment)
Moves the analysis to use the worklist strategy: rust-lang/rust#121421 (comment)
Also fixes rust-lang/rust#85836, using the same reachability analysis

Author: bors