SQL: Extend the optimisations for equalities (#50792)

[bpintea]

• Extend the optimizations for equalities

This commit supplements the optimisations of equalities in conjunctions
and disjunctions:

• for conjunctions, the existing optimizations with ranges are extended
  with not-equalities and inequalities; these lead to a fast resolution,
  the conjunction either being evaluate to a FALSE, or the non-equality
  conditions being dropped as superfluous;

• optimisations for disjunctions are added to be applied against ranges,
  inequalities and not-equalities; these lead to disjunction either
  becoming TRUE or the equality being dropped, either as superfluous or
  merged into a range/inequality.

• Adress review notes

• Fix the bug around wrongly optimizing 'a=2 OR a!=?', which only yields
  TRUE for same values in equality and inequality.

• Var renamings, code style adjustments, comments corrections.

• Address further review comments. Extend optim.

• fix a few code comments;
• extend the Equals OR NotEquals optimitsation (a=2 OR a!=5 -> a!=5);
• extend the Equals OR Range optimisation on limits equality (a=2 OR
  2<=a<5 -> 2<=a<5);
• in case an equality is being removed in a conjunction, the rest of
  possible optimisations to test is now skipped.

• rename one var for better legiblity

• s/rmEqual/removeEquals

(cherry picked from commit 62e7c6a)

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