nir: Lower returns correctly inside nested loops

nir: Lower returns correctly inside nested loops

Inside nested flow control, nir_lower_returns inserts predicated breaks
in the outer block. However, it would omit doing this if the remainder
of the outer block (after the inner block) was empty. This is not
correct in the case of loops, as execution just wraps back around to the
start of the loop, so this change doesn't skip the predication inside
loops.

Fixes: 79dec93ead6e (nir: Add return lowering pass)
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/2724

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4603>