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

llvm-svn: 233751