glsl: fix new gcc6 warnings

glsl: fix new gcc6 warnings

src/compiler/glsl/lower_discard_flow.cpp:79:1: warning: ‘ir_visitor_status {anonymous}::lower_discard_flow_visitor::visit_enter(ir_loop_jump*)’ defined but not used [-Wunused-function]
 lower_discard_flow_visitor::visit_enter(ir_loop_jump *ir)
 ^~~~~~~~~~~~~~~~~~~~~~~~~~

The base class method that was intended to be overridden was
'visit(ir_loop_jump *ir)', not visit_enter().

Signed-off-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>