nir: Generalize the optimization of subs of subs from 0.

I initially wrote this based on the "(('fneg', ('fneg', a)), a)" above,
but we can generalize it and make it more potentially useful.  In the
specific original case of a 0 for our new 'a' argument, it'll get further
algebraic optimization once the 0 is an argument to the new add.

No shader-db effects.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>

src/glsl/nir/nir_opt_algebraic.py

@@ -151,8 +151,8 @@ optimizations = [
    (('fcsel', a, b, b), b),
 
    # Subtracts
-   (('fsub', 0.0, ('fsub', 0.0, a)), a),
-   (('isub', 0, ('isub', 0, a)), a),
+   (('fsub', a, ('fsub', 0.0, b)), ('fadd', a, b)),
+   (('isub', a, ('isub', 0, b)), ('iadd', a, b)),
    (('fneg', a), ('fsub', 0.0, a), 'options->lower_negate'),
    (('ineg', a), ('isub', 0, a), 'options->lower_negate'),
    (('fadd', a, ('fsub', 0.0, b)), ('fsub', a, b)),