glsl: Make condition_to_hir() callable from outside ast_iteration_statement.

glsl: Make condition_to_hir() callable from outside ast_iteration_statement.

In addition to making it public, we also need to change its first
argument from an ir_loop * to an exec_list *, so that it can be used
to insert the condition anywhere in the IR (rather than just in the
body of the loop).

This will be necessary in order to make continue statements work
properly in do-while loops.

Cc: mesa-stable@lists.freedesktop.org
Acked-by: Carl Worth <cworth@cworth.org>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>