Executor no longer cares about mergejoinop, mergerightorder, mergeleftorder,

Executor no longer cares about mergejoinop, mergerightorder, mergeleftorder,
so remove them from MergeJoin node.  Hack together a partial
solution for commuted mergejoin operators --- yesterday
a mergejoin int4 = int8 would crash if the planner decided to
commute it, today it works.  The planner's representation of
mergejoins really needs a rewrite though.
Also, further testing of mergejoin ops in opr_sanity regress test.