poly: Add a poly_nir_lower_sysvals() pass

There are a few sysvals which exist just so we can specialize them based
on shader keys or linking.  In the case where we can't specialize them,
this provides a pass which loads them from the appropriate poly_*_param
struct.

Acked-by: Alyssa Rosenzweig <alyssa.rosenzweig@intel.com>
Reviewed-by: Mary Guillemard <mary@mary.zone>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/38404>

src/poly/nir/meson.build

@@ -3,6 +3,7 @@
 
 libpoly_nir_files = files(
   'poly_nir_lower_gs.c',
+  'poly_nir_lower_sysvals.c',
   'poly_nir_lower_tess.c',
   'poly_nir_lower_vs.c',
 )

src/poly/nir/poly_nir.h

@@ -59,3 +59,5 @@ bool poly_nir_lower_tes(struct nir_shader *tes, bool to_hw_vs);
 uint64_t poly_tcs_per_vertex_outputs(const struct nir_shader *nir);
 
 unsigned poly_tcs_output_stride(const struct nir_shader *nir);
+
+bool poly_nir_lower_sysvals(struct nir_shader *nir);

src/poly/nir/poly_nir_lower_sysvals.c

@@ -0,0 +1,31 @@
+/*
+ * Copyright 2023 Valve Corporation
+ * SPDX-License-Identifier: MIT
+ */
+
+#include "nir_builder.h"
+#include "poly/cl/libpoly.h"
+#include "poly/nir/poly_nir.h"
+
+static bool
+lower_sysvals_intr(nir_builder *b, nir_intrinsic_instr *intr, void *data)
+{
+   switch (intr->intrinsic) {
+   case nir_intrinsic_load_vs_outputs_poly: {
+      b->cursor = nir_before_instr(&intr->instr);
+      nir_def *vp = nir_load_vertex_param_buffer_poly(b);
+      nir_def_replace(&intr->def, poly_vertex_outputs(b, vp));
+      return true;
+   }
+
+   default:
+      return false;
+   }
+}
+
+bool
+poly_nir_lower_sysvals(struct nir_shader *nir)
+{
+   return nir_shader_intrinsics_pass(nir, lower_sysvals_intr,
+                                     nir_metadata_control_flow, NULL);
+}