Auto merge of #96815 - SparrowLii:promote_const, r=oli-obk

optimize `promote_consts` by caching the results of `validate_local`

From the FIXME in the impl of `promote_consts`. Early return the `validate_local` should save some compile time.

`qualif_local` is similar to this, but requires futher changing because there are different types of qualif checks. If this PR is effective, I will do it as well.

Author: bors