nir/lower_scratch: Ensure we don't lower vars with unsupported usage.

nir/lower_scratch: Ensure we don't lower vars with unsupported usage.

Need to avoid lowering temps when they are used by other instructions,
like the rt instructions (some of the shader call parameters get
converted to temp variables and we will lower them later with
the explicit io lowering pass as we need to guarantee they will
end up in scratch).

Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12162>