i965/spirv: Lower shared memory later

i965/spirv: Lower shared memory later

Instead of asking spirv_to_nir to lower the workgroup (shared memory)
to offsets, keep them as derefs longer, then lower it later on.

Because Workgroup memory doesn't have explicit offsets, we need to set
those using nir_lower_vars_to_explicit_types before calling the I/O
lowering pass.

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