[SSADestroyHoisting] Adopt new utilities.

Instead of doing one or two non-iterative BackwardReachability runs,
do a single run of IterativeBackwardReachability.  During that, pause
after discovery/local dataflow and use VisitBarrierAccessScopes to
determine which end_access instructions in the discovered region are
barriers.  Add those instructions as kills to the dataflow.  Finally run
the global dataflow.

Enables SSADestroyHoisting to hoist destroys over loops.

Addresses a correctness issue where access scopes which were open at
barrier blocks were not promoted to barriers, resulting in destroy_addrs
getting hoisted into unrelated access scopes.

Author: nate-chandler