nir: Insert phi undef sources while lowering returns

Alternate proposal to what I had in !22787 (merged). CI's happy with it, except that it obliviates a couple of tests for nir_lower_returns. Trivial phis no longer survive that pass, because if they survive to the point where the jump instruction is removed, they now gain an additional source which prevents them from being removed later if they were part of an if.