aco: Optimize MUBUF 0 offset when idxen is also being used.

aco: Optimize MUBUF 0 offset when idxen is also being used.

Now that we added an index src to the NIR intrinsic, it can
happen that these generate MUBUF instructions which have both
an index and an offset.

Extend this ACO optimization to the case when idxen is used.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Georg Lehmann <dadschoorse@gmail.com>
Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17551>