Whether to back-patch fix for aggregate transtype width estimates

While fixing the recent unpleasantness over parallel polymorphic
aggregates, I discovered that count_agg_clauses_walker's consideration
of an aggregate argument's typmod in estimating transition space
consumption has been broken since commit 34d26872e (which changed
Aggref.args from a simple expression list to a list of TargetEntry).
It had been looking at "exprTypmod((Node *) linitial(aggref->args))",
and nobody taught it that there was now a TargetEntry in the way that
it needed to look through to get a correct answer.

Ordinarily I'd just summarily back-patch a fix, but that commit shipped
in 9.0, which means it's been broken a long time. I'm worried that
back-patching a change might be more likely to destabilize plan choices
than to do anything anybody's happy about.

Thoughts?

regards, tom lane