when checking isNull(), we'd pick off the sentinel bit for the outer
PointerUnion, but would not recursively convert the inner pointerunion to bool,
so if *its* sentinel bit is set, isNull() would incorrectly return false.
No testcase, because someone hit this when they were trying to refactor code
to use PointerUnion3, but they since found a better solution.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@137428 91177308-0d34-0410-b5e6-96231b3b80d8
823eb1ca11