Correctly assess parallel-safety of tlists when SRFs are used.

Correctly assess parallel-safety of tlists when SRFs are used.

Since commit 69f4b9c85f168ae006929eec44fc44d569e846b9, the existing
code was no longer assessing the parallel-safety of the real tlist
for each upper rel, but rather the first of possibly several tlists
created by split_pathtarget_at_srfs().  Repair.

Even though this is clearly wrong, it's not clear that it has any
user-visible consequences at the moment, so no back-patch for now.  If
we discover later that it does have user-visible consequences, we
might need to back-patch this to v10.

Patch by me, per a report from Rajkumar Raghuwanshi.

Discussion: http://postgr.es/m/CA+Tgmoaob_Strkg4Dcx=VyxnyXtrmkV=ofj=pX7gH9hSre-g0Q@mail.gmail.com