condition we're unswitching on. In this case, don't try to
simplify the second copy of the loop which may be dead or not,
but is probably a constant now. This fixes PR6879
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@101870 91177308-0d34-0410-b5e6-96231b3b80d8