CScalarCmp and CScalarIsDistinctFrom operator must have the CScalarIdent operator
on the LHS and CScalarConst operator on the RHS. If it's
not the case the predicate is assigned a default selectivity
due to which the Cardinality estimate is impacted.

This patch fixes the issue by reordering the children of
CScalarCmp and CScalarIsDistinctFrom operator if CScalarIdent operator is on
the RHS on CScalarConst is on LHS. Also the Comparision operator is changed
due to the reordering of the arguments, if supported. If the
corresponding comparision operator does not exist or supported, the
children are not reordered.
Only cases of the type CONST = VAR are handled by this patch.
Signed-off-by: NOmer Arap <oarap@pivotal.io>