nir: update nir_lower_returns to only predicate instructions when needed

nir: update nir_lower_returns to only predicate instructions when needed

Unless an if statement contains nested returns we can simply add
any following instructions to the branch without the return.

V2: fix handling if_nested_return value when there is a sibling if/loop
that doesn't contain a return. (Spotted by Ken)

V3:
 - add a better comment to the new variable
 - remove instructions after if when both branches return

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