Fix a silly use-after-free issue. A much earlier version of this code

Fix a silly use-after-free issue. A much earlier version of this code
need lots of fanciness around retaining a reference to a Chain's slot in
the BlockToChain map, but that's all gone now. We can just go directly
to allocating the new chain (which will update the mapping for us) and
using it.

Somewhat gross mechanically generated test case replicates the issue
Duncan spotted when actually testing this out.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@145120 91177308-0d34-0410-b5e6-96231b3b80d8