nir/algebraic: fix iabs(ishr(iabs(a), b)) optimization

iabs(a) is not positive if "a" is the minimum signed value, so this is
incorrect in that case for some values of "b".

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Fixes: 2b76de9b5d ("nir/algebraic: Add a couple optimizations for iabs and ishr")
(cherry picked from commit ecd6ae12fb)

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/32261>

src/compiler/nir/nir_opt_algebraic.py

@@ -1320,7 +1320,8 @@ optimizations.extend([
    # negative.
    (('bcsel', ('ilt', a, 0), ('ineg', ('ishr', a, b)), ('ishr', a, b)),
     ('iabs', ('ishr', a, b))),
-   (('iabs', ('ishr', ('iabs', a), b)), ('ishr', ('iabs', a), b)),
+   (('iabs', ('ishr', ('iabs', a), b)), ('ushr', ('iabs', a), b)),
+   (('iabs', ('ushr', ('iabs', a), b)), ('ushr', ('iabs', a), b)),
 
    (('fabs', ('slt', a, b)), ('slt', a, b)),
    (('fabs', ('sge', a, b)), ('sge', a, b)),