allow builtin functions to require parameters to be shader inputs

The new interpolateAt* builtins have strange restrictions on the
<interpolant> parameter.

- It must be a shader input, or an element of a shader input array.
- It must not include a swizzle.

V2: Don't abuse ir_var_mode_shader_in for this; make a new flag.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>

src/glsl/ast_function.cpp

@@ -178,6 +178,24 @@ verify_parameter_modes(_mesa_glsl_parse_state *state,
 	 return false;
       }
 
+      /* Verify that shader_in parameters are shader inputs */
+      if (formal->data.must_be_shader_input) {
+         ir_variable *var = actual->variable_referenced();
+         if (var && var->data.mode != ir_var_shader_in) {
+            _mesa_glsl_error(&loc, state,
+                             "parameter `%s` must be a shader input",
+                             formal->name);
+            return false;
+         }
+
+         if (actual->ir_type == ir_type_swizzle) {
+            _mesa_glsl_error(&loc, state,
+                             "parameter `%s` must not be swizzled",
+                             formal->name);
+            return false;
+         }
+      }
+
       /* Verify that 'out' and 'inout' actual parameters are lvalues. */
       if (formal->data.mode == ir_var_function_out
           || formal->data.mode == ir_var_function_inout) {