nir/loop_analyze: Fix get_iteration for nir_op_fneu

nir/loop_analyze: Fix get_iteration for nir_op_fneu

Consider the loop:

    float i = 0.0;
    while (true) {
       if (i != 0.0)
          break;

       i = i + 1.0;
    }

This loop clearly executes exactly one time.

Some trickery is necessary to handle cases where the initial loop value
is very large and the increment is, by comparison, very small.  From the
fenu_once test case,

    float i = -604462909807314587353088.0;
    while (true) {
       if (i != -604462909807314587353088.0)
          break;

       i = i + 36028797018963968.0;
    }

This loop should also execute exactly once, but this is much more
challenging to calculate due to precision issues.

Going towards smaller magnitude (i.e., adding a small positive value to
a large negative value) requires a smaller delta to make a difference
than going towards a larger magnitude. For this reason,
-604462909807314587353088.0 + 36028797018963968.0 !=
-604462909807314587353088.0, but -604462909807314587353088.0 +
-36028797018963968.0 == -604462909807314587353088.0. Math class is
tough.

No changes in shader-db or fossil-db.

v2: Fix major bug in checking result of the eval_const_binop(nir_op_feq,
...) discovered while developing fneu_once_easy unit test. Fix a typo in
the comment just above that. Add fneu_once_easy test.

v3: Skip the iteration count adjustment tests for nir_op_fenu and
nir_op_ine. Since the iteration count is either 1 or unknown, all this
function can do is add numerical error. Add fenu_once tests.

v4: Change the initial value in the fneu_once test from large positive
to large negative. Change check in get_iteration from nir_op_fsub to
nir_op_fadd. Both changes from discussion with M Henning. Also add some
more explanation in fneu_once.

v5: Rename test cases.

Fixes: 6772a17acc8 ("nir: Add a loop analysis pass")
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19732>