release-19.2: opt: backport two pg compatibility fixes for GROUP BY#42694

Merged

RaduBerinde merged 3 commits intocockroachdb:release-19.2from RaduBerinde:backport19.2-41732-42447

Nov 26, 2019

+867-301

Commits on Nov 22, 2019

opt: clean up optbuilder group-by code
RaduBerinde
committed

Commits on Nov 23, 2019