sql: fix decimal evaluation edge cases

Previously, the logic for decimal and float division, floor division
and mod operators was incorrect for a few edge cases involving `NaN`
or `Infinity` values. For example, `'NaN'::DECIMAL / 0` would throw
a division-by-zero error when it should evaluate to `NaN` and
`0/'inf'::DECIMAL` returned `0E-2019` instead of just `0`.

This patch updates the special-case logic to mirror that of postgres,
so division-by-zero errors always check the `NaN` case and the division
by infinity case returns a `0` without extra digits.

Fixes cockroachdb#40929
Fixes cockroachdb#103633

Release note (bug fix): Fixed edge cases in decimal and float evaluation
for division operators. `'NaN'::DECIMAL / 0` will now return `NaN` instead
of a division-by-zero error, and `0 / 'inf'::DECIMAL` will return `0`
instead of `0E-2019`.