glsl: Use simpler visitor to determine which UBO and SSBO blocks are used

glsl: Use simpler visitor to determine which UBO and SSBO blocks are used

Very soon this visitor will get more complicated.  The users of the
existing ir_variable_refcount visitor won't need the coming
functionality, and this use doesn't need much of the functionality of
ir_variable_refcount.

v2: ir_array_refcount_visitor::get_variable_entry cannot return NULL, so
don't check it.  Suggested by Timothy.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>