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

I wrote:
> After contemplating that for awhile, by far the least painful way
> seems to be to add the subLinkId to struct SubPlan.  We can get
> away with that in the back branches as long as we add it at the
> end, since SubPlans don't get recorded in the catalogs.

The attached seems to fix it.  While writing a test case I discovered
that the code I'd stolen from ruleutils.c is inadequate for the purpose.
That code was only designed to deal with pre-planning query trees,
so it only expects one Param per targetlist entry --- but cases like
multiple assignments to the same array column can get rolled up into
nests containing several Params (cf. rewriteTargetListIU).  So I abandoned
that tactic in favor of just writing a tree walker.  Andre's gut feeling
that we needed one was right.

			regards, tom lane