IR: Metadata: Detect an RAUW recursion

Speculatively handle a recursion in
`GenericMDNode::handleChangedOperand()`.  I'm hoping this fixes the
failing hexagon bot [1].

[1]: http://lab.llvm.org:8011/builders/llvm-hexagon-elf/builds/13434

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223849 91177308-0d34-0410-b5e6-96231b3b80d8