nir: Correctly constant fold fsign(NaN) and fsign(-0)

nir: Correctly constant fold fsign(NaN) and fsign(-0)

GLSL and SPIR-V GLSL.std.450 don't have any requirements for fsign(NaN),
and both only require that FSign(-0.0) == 0.0.  OpenCL, on the other
hand, requires sign(-0.0) be exactly -0.0.  It also requires that
sign(NaN) be exactly 0.0.

In practice, this change is difficult to test.  Our GLSL frontend
already constant folds sign(NaN) to 0.0 before even getting to NIR.  As
far as I can tell, glslang does the same.  I don't have a good way to
run an OpenCL SPIR-V test.  Maybe SPIR-V GLSL.std.450 assembly?

No shader-db or fossil-db changes on any Intel platform.

Acked-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6358>