Auto merge of #154375 - RalfJung:large_enums, r=wesleywiser

large_enums mir pass: fix is_enabled logic

This logic got turned from a guard in `run_pass` to an `is_enabled` check in rust-lang/rust@15d4728#r2989096960, but that was done incorrectly. The pass must never be enabled without `unsound_mir_opts`!

Author: bors