ValueMapper: Only put cyclic nodes into CyclicNodes, NFCI

As a minor fixup to r266258, only track nodes that needed a placeholder
in CyclicNodes in MDNodeMapper::mapUniquedNodes.  There should be no
observable functionality change, just some local memory savings because
CyclicNodes only needs to grow to accommodate nodes that are actually
involved in cycles.  (This was the original intent of r266258, or else
the vector would have been called "ChangedNodes".)

llvm-svn: 266536