target/hexagon: idef-parser simplify predicate init

Only predicate instruction arguments need to be initialized by
idef-parser. This commit removes registers from the init_list and
simplifies gen_inst_init_args() slightly.

Signed-off-by: Anton Johansson <anjo@rev.ng>
Reviewed-by: Taylor Simpson <ltaylorsimpson@gmail.com>
Reviewed-by: Brian Cain <bcain@quicinc.com>
Message-Id: <20240523125901.27797-5-anjo@rev.ng>
Signed-off-by: Brian Cain <bcain@quicinc.com>