anv/pipeline: lower constant initializers on output variables earlier

anv/pipeline: lower constant initializers on output variables earlier

If a shader only writes to an output via a constant initializer we
need to lower it before we call nir_remove_dead_variables so that
this pass sees the stores from the initializer and doesn't kill the
output.

Fixes test failures in new work-in-progress CTS tests:
dEQP-VK.spirv_assembly.instruction.graphics.variable_init.output_vert
dEQP-VK.spirv_assembly.instruction.graphics.variable_init.output_frag

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>