glsl: Support if-flattening beyond a given maximum nesting depth.

glsl: Support if-flattening beyond a given maximum nesting depth.

This adds a new optional max_depth parameter (defaulting to 0) to
lower_if_to_cond_assign, and makes the pass only flatten if-statements
nested deeper than that.

By default, all if-statements will be flattened, just like before.

This patch also renames do_if_to_cond_assign to lower_if_to_cond_assign,
to match the new naming conventions.