Replace has_multiple_baserels() with a bitmap test on all_baserels.

Replace has_multiple_baserels() with a bitmap test on all_baserels.

Since we added the PlannerInfo.all_baserels set, it's not really
necessary to grovel over the rangetable to count baserels in the
current query.  So let's drop has_multiple_baserels() in favor
of a bms_membership() test.  This might be microscopically
faster, but the main point is to remove some unnecessary code.

Richard Guo

Discussion: https://postgr.es/m/CAMbWs4_8RcSbbfs1ASZLrMuL0c0EQgXWcoLTQD8swBRY_pQQiA@mail.gmail.com