Extend code that deduces implied equality clauses to detect whether a

Extend code that deduces implied equality clauses to detect whether a
clause being added to a particular restriction-clause list is redundant
with those already in the list.  This avoids useless work at runtime,
and (perhaps more importantly) keeps the selectivity estimation routines
from generating too-small estimates of numbers of output rows.
Also some minor improvements in OPTIMIZER_DEBUG displays.