spirv: Eliminate dead input/output variables after translation.

spirv_to_nir can generate input/output variables which are illegal
for the current shader stage, which would cause nir_validate_shader
to balk.  After my recent commit to start decorating arrays as compact,
dEQP-VK.spirv_assembly.instruction.graphics.module.same_module started
hitting validation errors due to outputs in a TCS (not intended for the
TCS at all) not being per-vertex arrays.

Thanks to Jason Ekstrand for suggesting this approach.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=109573


Fixes: ef99f4c8 compiler: Mark clip/cull distance arrays as compact before lowering.
Reviewed-by: Juan A. Suarez <jasuarez@igalia.com>
(cherry picked from commit 6775665e)