gallivm/sample: detect if rho is inf or nan and flush to zero.

When using cubemaps and the u/v values are 0, then this point can be arrived at with rho = nan, and if rho is NaN, then lod calculations end up at the max lod, whereas the spec suggests they should end up at the most negative lod.

This fixes dEQP-VK.glsl.texture_functions.query.texturequerylod.samplercube_float_zero_uv_width_fragment