glsl: Split out ir_reader's ability to read ir_dereference_variables.

glsl: Split out ir_reader's ability to read ir_dereference_variables.

Most of the time, we just want to read an ir_dereference, so there's no
need to have these in separate functions.  However, the next patch will
want to read an ir_dereference_variable directly.

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