IR: Drop uniquing for self-referencing MDNodes

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