nir: Lower !f2b(x) to x == 0.0

Some trivial help now, but it also prevents ~40 regressions caused by
Samuel's "nir: implement the GLSL equivalent of if simplication in
nir_opt_if" patch.

All Gen4+ platforms had similar results. (Skylake shown)
total instructions in shared programs: 14369557 -> 14369555 (<.01%)
instructions in affected programs: 442 -> 440 (-0.45%)
helped: 2
HURT: 0

total cycles in shared programs: 532425772 -> 532425743 (<.01%)
cycles in affected programs: 6086 -> 6057 (-0.48%)
helped: 2
HURT: 0

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>

src/compiler/nir/nir_opt_algebraic.py

@@ -441,6 +441,7 @@ optimizations = [
    (('i2b', ('iabs', a)), ('i2b', a)),
    (('fabs', ('b2f', a)), ('b2f', a)),
    (('iabs', ('b2i', a)), ('b2i', a)),
+   (('inot', ('f2b', a)), ('feq', a, 0.0)),
 
    # Packing and then unpacking does nothing
    (('unpack_64_2x32_split_x', ('pack_64_2x32_split', a, b)), a),