glsl: Add IR builder support for generating return statements.

glsl: Add IR builder support for generating return statements.

We use "ret" as the function name since "return" is a C++ keyword, and
"ir_return" is already a class name.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Paul Berry <stereotype441@gmail.com>