nir: gather if a fragment shader uses sample shading

This introduces a new flag in shader_info to know if a fragment
shader uses sample shading, even if there is no inputs.

During NIR linking, constants varyings are optimized and the
per-sample interpolation info (ie. the sample qualifier) might
be removed if nir_shader_gather_info() is called again.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7876>

src/compiler/shader_info.h

@@ -286,6 +286,11 @@ typedef struct shader_info {
           */
          bool uses_sample_qualifier:1;
 
+         /**
+          * Whether sample shading is used.
+          */
+         bool uses_sample_shading:1;
+
          /**
           * Whether early fragment tests are enabled as defined by
           * ARB_shader_image_load_store.