gallivm: avoid division by zero when computing cube face

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

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15246>