It doesn't make sense to unique self-referencing nodes. Drop uniquing
for them.
Note that `MDNode::intersect()` occasionally returns self-referencing
nodes. Previously these would be returned by `MDNode::get()`. I'm not
convinced this was intended behaviour -- to me it seems it should return
a node whose only operand is the self-reference -- but I don't know much
about alias scopes so I'm preserving it for now.
This is part of PR21532.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223618 91177308-0d34-0410-b5e6-96231b3b80d8