nir/algebraic: Optimize x2b(xneg(a)) -> a

Shader-db results on Kaby Lake:

    total instructions in shared programs: 15072525 -> 15072525 (0.00%)
    instructions in affected programs: 0 -> 0
    helped: 0
    HURT: 0

This helps prevent regressions in later commits.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Tested-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>

src/compiler/nir/nir_opt_algebraic.py

@@ -85,6 +85,8 @@ optimizations = [
    (('fabs', ('u2f', a)), ('u2f', a)),
    (('iabs', ('iabs', a)), ('iabs', a)),
    (('iabs', ('ineg', a)), ('iabs', a)),
+   (('f2b', ('fneg', a)), ('f2b', a)),
+   (('i2b', ('ineg', a)), ('i2b', a)),
    (('~fadd', a, 0.0), a),
    (('iadd', a, 0), a),
    (('usadd_4x8', a, 0), a),