nir/validate: Improve the validation of blocks

nir/validate: Improve the validation of blocks

This commit adds a number of new validation checks:

 1. We now check that every block pointer in the IR points to a block
    that actually exists in a block list that's reachable from the
    nir_function_impl.

 2. We assert that nir_function_impl::body is non-empty

 3. We assert that the start block has no predecessors.  This is
    important because we tend to put run-once code there.

 4. We now validate some stuff on the end block.

Reviewed-by: Karol Herbst <kherbst@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6750>