IR: Enable uniquing callbacks during MDNode::replaceWithUniqued()

IR: Enable uniquing callbacks during MDNode::replaceWithUniqued()

Uniqued nodes have more complete registration with
`ReplaceableMetadataImpl` so that they can update themselves when
operands change.  Fix a bug where `MDNode::replaceWithUniqued()` wasn't
enabling these callbacks.

The two most obvious ways missing callbacks causes problems is that
auto-resolution fails and re-uniquing (on changed operands) just doesn't
happen.  I've added tests for both -- in both cases, I confirmed that
the final check was failing before the fix.

rdar://problem/20365935

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@233751 91177308-0d34-0410-b5e6-96231b3b80d8