Rollup merge of #105683 - JakobDegen:dest-prop-storage, r=tmiasko

Rollup merge of #105683 - JakobDegen:dest-prop-storage, r=tmiasko

Various cleanups to dest prop

This makes fixing the issues identified in #105577 easier. A couple changes

 - Use an enum with names instead of a bool
 - Only call `remove_candidates_if` from one place instead of two. Doing it from two places is far too fragile, since any divergence in the behavior between those callsites is likely to be unsound.
 - Remove `is_constant`. Right now we only merge locals, so this doesn't do anything, and the logic would be wrong if it did.

r? `@tmiasko`