nir,ac/llvm: add nir_intrinsic_load_half_line_width_amd

Used by AMD GPU NGG line culling. We could use nir load
line width and viewport scale to calculate this in shader,
but this way needs expensive divide ops.

Acked-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Timur Kristóf <timur.kristof@gmail.com>
Signed-off-by: Qiang Yu <yuq825@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17651>

src/compiler/nir/nir_divergence_analysis.c

@@ -181,6 +181,7 @@ visit_intrinsic(nir_shader *shader, nir_intrinsic_instr *instr)
    case nir_intrinsic_load_ray_num_dss_rt_stacks_intel:
    case nir_intrinsic_load_lshs_vertex_stride_amd:
    case nir_intrinsic_load_hs_out_patch_data_offset_amd:
+   case nir_intrinsic_load_clip_half_line_width_amd:
       is_divergent = false;
       break;