pan/bi: Don't allow ATEST to take a temporary

pan/bi: Don't allow ATEST to take a temporary

Clause scheduler edition of db2bdc1dc37 ("pan/bi: Require ATEST coverage mask
input in R60"). ATEST wants to read r60, which can't work if its input isn't
even in a register.

When per-sample shading isn't in use, prevents regressions in:

   KHR-GLES31.core.sample_variables.mask.*

These tests previously passed because per-sample shading was forced. It's
not clear whether the bug addressed in this patch is possible to hit "in the
wild", i.e. without the optimizations in this series that allow us to use
per-pixel shading in more cases.

No shader-db changes.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Cc: mesa-stable
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17841>