refactor: split ATExecAlterConstrRecurse()

refactor: split ATExecAlterConstrRecurse()

This splits out a couple of subroutines from
ATExecAlterConstrRecurse().  This makes the main function a bit
smaller, and a future patch (NOT ENFORCED foreign-key constraints)
will also want to call some of the pieces separately.

Author: Amul Sul <[email protected]>
Reviewed-by: jian he <[email protected]>
Discussion: https://www.postgresql.org/message-id/flat/CAAJ_b962c5AcYW9KUt_R_ER5qs3fUGbe4az-SP-vuwPS-w-AGA%40mail.gmail.com