glsl: Make ir_validate actually visit ir_if nodes.

glsl: Make ir_validate actually visit ir_if nodes.

There is no ir_hierarchical_visitor::visit(ir_if *) method, since ir_if
is not a leaf node.  Instead, there are visit_enter and visit_leave
methods.  Use visit_enter arbitrarily (either would work fine, though
visit_enter will catch errors sooner).

Found thanks to a warning emitted by Clang.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>