nir: Add a lowering pass for polygon and line smoothing

When poly_line smoothing is enabled then this lowering pass will
modify the alpha component of every write to fragment output
using sample coverage mask.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Jason Ekstrand <jason.ekstrand@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16245>

src/compiler/nir/meson.build

@@ -187,6 +187,7 @@ files_libnir = files(
   'nir_lower_pntc_ytransform.c',
   'nir_lower_point_size.c',
   'nir_lower_point_size_mov.c',
+  'nir_lower_poly_line_smooth.c',
   'nir_lower_printf.c',
   'nir_lower_regs_to_ssa.c',
   'nir_lower_readonly_images_to_tex.c',