large_enums mir pass: fix is_enabled logic

[RalfJung]

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

[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[rustbot]

r? @mati865

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