aco/ra: use get_reg_specified() for p_extract_vector

On GFX6/7, it might violate validation rules, otherwise.

Fixes: 51f4b22feec3720c89458094a3245efc984115ee ('aco: don't allow unaligned subdword accesses on GFX6/7')
Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8047>
(cherry picked from commit fd49ba59a3888795ad0788259e3472e08c42bc24)
2 files changed