There are some situations which can cause the URoR hack to infinitely recurse

There are some situations which can cause the URoR hack to infinitely recurse
and then go kablooie. The problem was that it was tracking the PHI nodes anew
each time into this function. But it didn't need to. And because the recursion
didn't know that a PHINode was visited before, it would go ahead and call
itself.

There is a testcase, but unfortunately it's too big to add. This problem will go
away with the EH rewrite.
<rdar://problem/8856298>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@127640 91177308-0d34-0410-b5e6-96231b3b80d8