i965/vec4: Use NIR remapping for VS attributes

The NIR pass already handles remapping system values to attributes for
us so we delete the system value code as part of the conversion.

We also change nir_lower_vs_inputs to take an explicit inputs_read
bitmask and pass in the inputs_read from prog_data instead from pulling
it out of NIR.  This is because the version in prog_data may get
EDGEFLAG added to it on some old platforms.

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/intel/compiler/brw_vec4_vs_visitor.cpp

@@ -36,35 +36,8 @@ vec4_vs_visitor::emit_prolog()
 dst_reg *
 vec4_vs_visitor::make_reg_for_system_value(int location)
 {
-   /* VertexID is stored by the VF as the last vertex element, but
-    * we don't represent it with a flag in inputs_read, so we call
-    * it VERT_ATTRIB_MAX, which setup_attributes() picks up on.
-    */
-   dst_reg *reg = new(mem_ctx) dst_reg(ATTR, VERT_ATTRIB_MAX);
-
-   switch (location) {
-   case SYSTEM_VALUE_BASE_VERTEX:
-      reg->writemask = WRITEMASK_X;
-      break;
-   case SYSTEM_VALUE_BASE_INSTANCE:
-      reg->writemask = WRITEMASK_Y;
-      break;
-   case SYSTEM_VALUE_VERTEX_ID:
-   case SYSTEM_VALUE_VERTEX_ID_ZERO_BASE:
-      reg->writemask = WRITEMASK_Z;
-      break;
-   case SYSTEM_VALUE_INSTANCE_ID:
-      reg->writemask = WRITEMASK_W;
-      break;
-   case SYSTEM_VALUE_DRAW_ID:
-      reg = new(mem_ctx) dst_reg(ATTR, VERT_ATTRIB_MAX + 1);
-      reg->writemask = WRITEMASK_X;
-      break;
-   default:
-      unreachable("not reached");
-   }
-
-   return reg;
+   unreachable("not reached");
+   return NULL;
 }