Revamp handling of labels. In particular, if we create a forward reference

Revamp handling of labels.  In particular, if we create a forward reference
for a basic block, use it when the block is defined instead of deleting it
and creating a new one.  Also, only create at most ONE forward reference
for any block, instead of one for each forward reference.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@14807 91177308-0d34-0410-b5e6-96231b3b80d8