Back-patch "Fix parallel hash join path search."

Back-patch "Fix parallel hash join path search."

Back-patch commit 655393a022bd653e2b48dbf20b69236981e35195 to 9.6.  This
synchronizes the relavant code and helps in generating parallel paths in
some cases in 9.6.  This also helps in back-patch of future patches where
we can get the same plan in all branches.

Discussion: https://postgr.es/m/CAA4eK1+XK_875cJA1HPVpx9C7C8Fp7i4QzLJ17T3igfU2iadxQ@mail.gmail.com