IR: Add test for handleChangedOperand() recursion

Turns out this can happen.  Remove the `FIXME` and add a testcase that
crashes without the extra logic.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@225657 91177308-0d34-0410-b5e6-96231b3b80d8

lib/IR/Metadata.cpp

@@ -537,8 +537,6 @@ void GenericMDNode::handleChangedOperand(void *Ref, Metadata *New) {
   if (InRAUW) {
     // We just hit a recursion due to RAUW.  Set the operand and move on, since
     // we're about to be deleted.
-    //
-    // FIXME: Can this cycle really happen?
     setOperand(Op, New);
     return;
   }