We now update pred lists during loop unrolling, rather than recomputing
them from scratch.

There are several parts to the fix: first, `optRedirectBlock' now has
a new ability to add pred references for the flow from a newly cloned
block, be it either to a remapped successor or a non-remapped successor.
Along with this we no longer copy over the block ref count in `CloneBlockState`.
These changes allow us to create the right pred links and ref counts in the
interior of a cloned subgraph.

Second, we now scrub block references from the original loop body blocks
instead of just setting their ref counts to zero.

Finally, we fix up references for exterior flow into and out of the unroll
complex.

Addresses one of the cases mentioned in #49030.