Fix PR8161, in which an unreachable loop causes recursive instruction simplification...

Fix PR8161, in which an unreachable loop causes recursive instruction simplification to try
to replace an instruction with itself.  Add a predicate to the simplifier to prevent this case.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@114097 91177308-0d34-0410-b5e6-96231b3b80d8