If the phi node was used by an unreachable instruction that ends up using

If the phi node was used by an unreachable instruction that ends up using
itself without going via a phi node then we could return false here in
spite of making a change.  Also, tweak the comment because this method
can (and always could) return true without deleting the original phi node.
For example, if the phi node was used by a read-only invoke instruction
which is used by another phi node phi2 which is only used by and only uses
the invoke, then phi2 would be deleted but not the invoke instruction and
not the original phi node.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@126129 91177308-0d34-0410-b5e6-96231b3b80d8