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

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>