intel/compiler: don't propagate cmp to add if add is saturated

From the Kaby Lake PRM Vol. 7 "Assigning Conditional Flags":

   * Note that the [post condition signal] bits generated at
     the output of a compute are before the .sat.

Paragraph about post_zero does not mention saturation, but
testing it on actual GPUs shows that conditional modifiers
are applied after saturation.

   * post_zero bit: This bit reflects whether the final
     result is zero after all the clamping, normalizing,
     or format conversion logic.

For signed types we don't care about saturation: it won't
change the result of conditional modifier.

For floating and unsigned types there two special cases,
when we can remove inst even if scan_inst is saturated: G
and LE. Since conditional modifiers are just comparations
against zero, saturating positive values to the upper
limit never changes the result of comparation.

For negative values:
(sat(x) >  0) == (x >  0) --- false
(sat(x) <= 0) == (x <= 0) --- true

Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/2610
Signed-off-by: Yevhenii Kolesnikov <yevhenii.kolesnikov@globallogic.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4167>