BitcodeWriter: Break recursion when enumerating Metadata, almost NFC

Use a worklist instead of recursing through MDNode operands in
ValueEnumerator.  The actual record output order has changed slightly,
but otherwise there's no functionality change.

I had to update test/Bitcode/metadata-function-blocks.ll.  I renumbered
nodes so they continue to match the implicit record ids.

llvm-svn: 266709