Reapply "[EquivalenceClasses] Replace findValue with contains (NFC)." This reverts the revert commit 616f447fc84bdc7655117f1b303d895dc3b93e4d. It includes updates to remaining users in Polly and Clang, to avoid failures when building those projects. GitOrigin-RevId: 32f24029c72dae175c9e2cc81f931f065a2ba347