glsl: Bail after reporting an error for non-constant const_in parameters.

Otherwise we continue and hit the "Illegal formal parameter mode"
assertion.

Fixes negative compile test texelFetchOffset.frag in piglit.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>

src/glsl/ast_function.cpp

@@ -164,6 +164,7 @@ match_function_by_name(exec_list *instructions, const char *name,
 	    _mesa_glsl_error(loc, state,
 			     "parameter `%s' must be a constant expression",
 			     formal->name);
+	    return ir_call::get_error_instruction(ctx);
 	 }
 
 	 if ((formal->mode == ir_var_out)