glsl: Factor out code which emits a new function into the IR stream.

A future commit will use the newly created function in a second place.

src/glsl/ast.h

@@ -714,4 +714,8 @@ _mesa_ast_field_selection_to_hir(const ast_expression *expr,
 				 exec_list *instructions,
 				 struct _mesa_glsl_parse_state *state);
 
+void
+emit_function(_mesa_glsl_parse_state *state, exec_list *instructions,
+	      ir_function *f);
+
 #endif /* AST_H */