We want to use the local_size when available to calculate the threadsize
in ir3, and we need it to work with e.g. computerator where we don't
have a nir shader. Add a local_size field and use that in computerator
instead of of a separate structure that's inaccessable to core ir3.

Also set a dummy local_size in the tests to avoid a divide-by-zero.

Part-of: <mesa/mesa!9498>