More PostgreSQL expression index compare fixes

More PostgreSQL expression index compare fixes

Additional fixes to PostgreSQL expression index compare feature.
The compare now correctly accommodates casts and differences in
spacing.
Added detection logic for operation clauses inside the expression,
skipping the compare of these expressions.
To accommodate these changes the logic for the comparison of the
indexes and unique constraints was moved to the dialect
implementation, allowing greater flexibility.

Co-authored-by: Mike Bayer <mike_mp@zzzcomputing.com>
Fixes: #1321
Fixes: #1327
Fixes: #1356
Change-Id: Icad15bc556a63bfa55b84779e7691c745d943c63