get_memoize_path: Don't exit quickly when PGS_NESTLOOP_PLAIN is unset.

get_memoize_path: Don't exit quickly when PGS_NESTLOOP_PLAIN is unset.

This function exits early in the case where the number of inner rows
is estimated to be less than 2, on the theory that in that case a
Nested Loop with inner Memoize must lose to a plain Nested Loop.
But since commit 4020b370f214315b8c10430301898ac21658143f it's
possible for a plain Nested Loop to be disabled, while a Nested Loop
with inner Memoize is still enabled. In that case, this reasoning
is not valid, so adjust the code not to exit early in that case.

This issue was revealed by a test_plan_advice failure on buildfarm
member skink, where NESTED_LOOP_MEMOIZE() couldn't be enforced on
replanning due to this early exit.

Discussion: http://postgr.es/m/CA+TgmoZUN8FT1Ah=m6Uis5bHa4FUa+_hMDWtcABG17toEfpiUg@mail.gmail.com