[Local] Zap blockaddress without users in ConstantFoldTerminator.

[Local] Zap blockaddress without users in ConstantFoldTerminator.

If the blockaddress is not destoryed, the destination block will still
be marked as having its address taken, limiting further transformations.

I think there are other places where the dead blockaddress constants are kept
around, I'll look into that as follow up.

Reviewers: craig.topper, brzycki, davide

Reviewed By: brzycki, davide

Differential Revision: https://reviews.llvm.org/D64936

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@366633 91177308-0d34-0410-b5e6-96231b3b80d8