Backward inter-shader code motion turns ALU results into outputs,
which led to getting IO with unsupported bit sizes. This prevents
that.
There is a new NIR option flag that indicates 16-bit support.
Fixes: c66967b5cb - nir: add nir_opt_varyings, new pass optimizing and compacting varyings
Reviewed-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/28431>