radv/ac: eliminate unused vertex shader outputs. (v2)

This is ported from radeonsi, and I can see at least one
Talos shader drops an export due to this, and saves some
VGPR usage.

v2: use shared code.

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Signed-off-by: Dave Airlie <airlied@redhat.com>
3 files changed