IR: Unify code for MDNode::isResolved(), NFC

Unify the definitions of `MDNode::isResolved()` and
`UniquableMDNode::isResolved()`.  Previously, `UniquableMDNode` could
answer this question more efficiently, but now that RAUW support has
been unified with `MDNodeFwdDecl`, `MDNode` doesn't need any casts to
figure out the answer.

llvm-svn: 226485