glsl: Move common code out of opt_algebraic's handle_expression().

glsl: Move common code out of opt_algebraic's handle_expression().

Matt and I had each screwed up these common required patterns recently, in
ways that wouldn't have been noticed for a long time if not for code
review.  Just enforce it in the caller so that we don't rely on code
review catching these bugs.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>