glsl/hierarchical_visitor: Call leave_callback on leaf nodes

glsl/hierarchical_visitor: Call leave_callback on leaf nodes

Previously for leaf ir_instructions only the enter callback was
called. This makes it a bit difficult to make a pass that wants to
visit every instruction using a stack. Making it call the leave
callback as well makes it behave less surprisingly.

Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Acked-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3885>