Fix mishandling of sortgroupref labels while splitting SRF targetlists.

Fix mishandling of sortgroupref labels while splitting SRF targetlists.

split_pathtarget_at_srfs() neglected to worry about sortgroupref labels
in the intermediate PathTargets it constructs.  I think we'd supposed
that their labeling didn't matter, but it does at least for the case that
GroupAggregate/GatherMerge nodes appear immediately under the ProjectSet
step(s).  This results in "ERROR: ORDER/GROUP BY expression not found in
targetlist" during create_plan(), as reported by Rajkumar Raghuwanshi.

To fix, make this logic track the sortgroupref labeling of expressions,
not just their contents.  This also restores the pre-v10 behavior that
separate GROUP BY expressions will be kept distinct even if they are
textually equal().

Discussion: https://postgr.es/m/CAKcux6=1_Ye9kx8YLBPmJs_xE72PPc6vNi5q2AOHowMaCWjJ2w@mail.gmail.com