With the advent of SPIR-V subgroup operations, compute shaders will have
to be slightly different depending on the SIMD size at which they
execute.  In order to allow us to do dispatch-width specific things in
NIR, we re-run the final NIR stages for each sIMD width.

One side-effect of this change is that we start rallocing fs_visitors
which means we need DECLARE_RALLOC_CXX_OPERATORS.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>