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 #40929
Fixes #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`.

pkg/cmd/roachtest/tests/sqlsmith.go

@@ -247,9 +247,6 @@ WITH into_db = 'defaultdb', unsafe_restore_incompatible_version;
  for _, exp := range []string{
  // Optimizer panic-injection surfaces as an internal error.
  "injected panic in optimizer",
- // TODO(yuzefovich): we temporarily ignore internal errors
- // that are because of #40929.
- "could not parse \"0E-2019\" as type decimal",
  } {
  expectedError = expectedError || strings.Contains(es, exp)
  }