nir/lower_io: add nir_io_semantics::interp_explicit_strict

This preserves the misnamed "per_vertex" flag in lowered IO.

Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/28312>

src/compiler/nir/nir.h

@@ -1950,7 +1950,8 @@ typedef struct nir_io_semantics {
    unsigned no_varying : 1;       /* whether this output isn't consumed by the next stage */
    unsigned no_sysval_output : 1; /* whether this system value output has no
                                      effect due to current pipeline states */
-   unsigned _pad : 2;
+   unsigned interp_explicit_strict : 1; /* preserve original vertex order */
+   unsigned _pad : 1;
 } nir_io_semantics;
 
 /* Transform feedback info for 2 outputs. nir_intrinsic_store_output contains

src/compiler/nir/nir_lower_io.c

@@ -340,6 +340,11 @@ emit_load(struct lower_io_state *state,
          var->data.precision == GLSL_PRECISION_MEDIUM ||
          var->data.precision == GLSL_PRECISION_LOW;
       semantics.high_dvec2 = high_dvec2;
+      /* "per_vertex" is misnamed. It means "explicit interpolation with
+       * the original vertex order", which is a stricter version of
+       * INTERP_MODE_EXPLICIT.
+       */
+      semantics.interp_explicit_strict = var->data.per_vertex;
       nir_intrinsic_set_io_semantics(load, semantics);
    }