Implement support for the case when a reference to a addr-of-bb

Implement support for the case when a reference to a addr-of-bb
label is generated, but then the block is deleted.  Since the
value is undefined, we just emit the label right after the entry
label of the function.  It might matter that the label is in the
same section as the function was afterall.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@98579 91177308-0d34-0410-b5e6-96231b3b80d8