[SGC] Complete deadend-reachable lifetimes.
For a function to have complete lifetimes, the lifetime of every def in
the function which is backwards-reachable from a dead-end block must be
completed.
Previously, every def in every block which appears in the post-order of
the function's blocks is completed. This was not enough: defs in
"unreachable blocks" (i.e. those which aren't forwards-reachable from
the function entry) must be completed too, and such blocks do not appear
in the function's post-order.
Here, defs in unreachable blocks are be completed too. Such defs must
also be completed in relative post-order. To do this, roots for the
non-entry post-orders must be found.
rdar://141197164
