nir: add nir_intrinsic_load_per_primitive_input, split from io_semantics flag

Instead of having 1 bit in nir_io_semantics indicating a per-primitive
FS input, add a dedicated intrinsic for it.

Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/29895>

src/intel/compiler/brw_fs_nir.cpp

@@ -4209,7 +4209,8 @@ fs_nir_emit_fs_intrinsic(nir_to_brw_state &ntb,
       break;
    }
 
-   case nir_intrinsic_load_input: {
+   case nir_intrinsic_load_input:
+   case nir_intrinsic_load_per_primitive_input: {
       /* In Fragment Shaders load_input is used either for flat inputs or
        * per-primitive inputs.
        */