Better heuristic for str.code/re.range (#3220)

Better heuristic for str.code/re.range (#3220)

To make sure that our `str.code` function is injectve (except for -1 in
the codomain), we send the inference that `str.code(x) == -1 v
str.code(x) != str.code(y) v x == y` for each pair of `str.code` terms.
Because of the order of disjuncts, `str.code(x) != str.code(y)` was
usually assigned true. This in turn lead to a difficult problem for the
arithmetic engine if there were more `str.code` applications than the
size of the domain. E.g. if we had `0 <= str.code(xi) < 10` for 0 <= i
<= 10, then the arithmetic engine had a difficult time finding a
conflict. This PR improves the heuristic by setting the phase of
`str.code(x) != str.code(y)` to false, so we prefer to keep the
`str.code` values equal instead of trying to make them different.

This change is also reflected in the models produced for inputs
involving `str.code`: Previously, we were producing models with
different values for the `str.code` whereas now the models are much more
uniform.

The PR adds two regressions, one testing `str.code` performance directly
and one testing it for `str.code` terms generated by `re.range`.

Signed-off-by: Andres Noetzli <anoetzli@amazon.com>