IR: Cleanup MDNode field use, NFC

Swap usage of `SubclassData32` and `MDNodeSubclassData`, and rename
`MDNodeSubclassData` to `NumUnresolved`.  Small drive-by cleanup to
`countUnresolvedOperands()` since otherwise the name clash with local
vars named `NumUnresolved` would be confusing.

llvm-svn: 226523