gallivm: avoid division by zero when computing cube face
this is illegal and produces NaNs which blow up the sample instr
cc: mesa-stable
fixes (llvmpipe and zink): KHR-GL45.incomplete_texture_access.sampler dEQP-GLES31.functional.program_uniform.by_pointer.render.array_in_struct.sampler2D_samplerCube_both dEQP-GLES31.functional.program_uniform.by_pointer.render.array_in_struct.sampler2D_samplerCube_fragment dEQP-GLES31.functional.program_uniform.by_pointer.render.array_in_struct.sampler2D_samplerCube_vertex dEQP-GLES31.functional.program_uniform.by_pointer.render.basic.samplerCube_both dEQP-GLES31.functional.program_uniform.by_pointer.render.basic.samplerCube_fragment dEQP-GLES31.functional.program_uniform.by_pointer.render.basic.samplerCube_vertex dEQP-GLES31.functional.program_uniform.by_pointer.render.basic_struct.sampler2D_samplerCube_both dEQP-GLES31.functional.program_uniform.by_pointer.render.basic_struct.sampler2D_samplerCube_fragment dEQP-GLES31.functional.program_uniform.by_pointer.render.nested_structs_arrays.sampler2D_samplerCube_both dEQP-GLES31.functional.program_uniform.by_pointer.render.nested_structs_arrays.sampler2D_samplerCube_fragment dEQP-GLES31.functional.program_uniform.by_pointer.render.nested_structs_arrays.sampler2D_samplerCube_vertex dEQP-GLES31.functional.program_uniform.by_pointer.render.struct_in_array.sampler2D_samplerCube_both dEQP-GLES31.functional.program_uniform.by_pointer.render.struct_in_array.sampler2D_samplerCube_fragment dEQP-GLES31.functional.program_uniform.by_pointer.render.struct_in_array.sampler2D_samplerCube_vertex dEQP-GLES31.functional.program_uniform.by_value.render.array_in_struct.sampler2D_samplerCube_both dEQP-GLES31.functional.program_uniform.by_value.render.array_in_struct.sampler2D_samplerCube_fragment dEQP-GLES31.functional.program_uniform.by_value.render.array_in_struct.sampler2D_samplerCube_vertex dEQP-GLES31.functional.program_uniform.by_value.render.basic.samplerCube_both dEQP-GLES31.functional.program_uniform.by_value.render.basic.samplerCube_fragment dEQP-GLES31.functional.program_uniform.by_value.render.basic.samplerCube_vertex dEQP-GLES31.functional.program_uniform.by_value.render.basic_struct.sampler2D_samplerCube_both dEQP-GLES31.functional.program_uniform.by_value.render.basic_struct.sampler2D_samplerCube_fragment dEQP-GLES31.functional.program_uniform.by_value.render.nested_structs_arrays.sampler2D_samplerCube_both dEQP-GLES31.functional.program_uniform.by_value.render.nested_structs_arrays.sampler2D_samplerCube_fragment dEQP-GLES31.functional.program_uniform.by_value.render.nested_structs_arrays.sampler2D_samplerCube_vertex dEQP-GLES31.functional.program_uniform.by_value.render.struct_in_array.sampler2D_samplerCube_both dEQP-GLES31.functional.program_uniform.by_value.render.struct_in_array.sampler2D_samplerCube_fragment dEQP-GLES31.functional.program_uniform.by_value.render.struct_in_array.sampler2D_samplerCube_vertex