nir: Remove sRGB colorspace conversion round-trip.

Some shaders in Civilization V and Beyond Earth do

   pow(pow(x, 2.2), 0.454545)

which is converting to and from sRGB colorspace.

A more general rule that replaces pow(pow(a, b), c) with pow(a, b * c)
actually regresses two shaders in Sun Temple in which the result of the
inner pow is used twice, once by another pow and once by another
instruction. Also, since 2.2 * 0.454545 isn't exactly one, the more
general pattern would have still left us with a pow, and I'm 2.2 *
0.454545 percent sure that's not what they want.

instructions in affected programs:     934 -> 886 (-5.14%)
helped:                                16

src/glsl/nir/nir_opt_algebraic.py

@@ -156,6 +156,8 @@ optimizations = [
    (('fpow', a, 2.0), ('fmul', a, a)),
    (('fpow', a, 4.0), ('fmul', ('fmul', a, a), ('fmul', a, a))),
    (('fpow', 2.0, a), ('fexp2', a)),
+   (('fpow', ('fpow', a, 2.2), 0.454545), a),
+   (('fpow', ('fabs', ('fpow', a, 2.2)), 0.454545), ('fabs', a)),
    (('fsqrt', ('fexp2', a)), ('fexp2', ('fmul', 0.5, a))),
    (('frcp', ('fexp2', a)), ('fexp2', ('fneg', a))),
    (('frsq', ('fexp2', a)), ('fexp2', ('fmul', -0.5, a))),