IR: Drop newline from AssemblyWriter::printMDNodeBody()

Remove a newline from `AssemblyWriter::printMDNodeBody()`, and add one
to `AssemblyWriter::writeMDNode()`.  NFCI for assembly output.

However, this drops an inconsistent newline from `Metadata::print()`
when `this` is an `MDNode`.  Now the newline added by `Metadata::dump()`
won't look so verbose.

llvm-svn: 230565