BUG #17606: There is still some glitch in 3f7323cbb fixing failure of MULTIEXPR_SUBLINK

The following bug has been logged on the website:

Bug reference:      17606
Logged by:          Andre Lin
Email address:      857348270@qq.com
PostgreSQL version: 13.8
Operating system:   Linux x86_64 GNU/Linux
Description:        

3f7323cbb regenerates its Param for each SubPlan by traversing the
targetlist. But ignore one point: initplan is not in targetlist.
This will result in a failed assertion, or an error like "unexpected
PARAM_MULTIEXPR ID: 131074"

reproduce it is simple, change the regress sql in inherit.sql to

explain (verbose, costs off)
update inhpar set (f1, f2[1]) = (select 1, '1' from int4_tbl limit 1)
from onek p2 where inhpar.f1 = p2.unique1;