aco: fix GFX8 16-bit packing

def.physReg() was uninitialized.

Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Fixes: d96f387e7ac448b450091b900ab089eea3eb7b86 ('aco: improve code sequences for 16bit packing')
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7334>
1 file changed