nir/dead_cf: add support for removing useless loops

nir/dead_cf: add support for removing useless loops

v2: fix detecting if the loop has any phi nodes after it.
v2: use nir_foreach_ssa_def() instead of nir_foreach_dest() when
    checking for values live after the loop to catch const_load
    instructions.
v2: fix handling return instructions
v2: add some documentation to loop_is_dead()

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>