Allow TableGen DAG arguments to be just a name.

DAG arguments can optionally be named:

  (dag node, node:$name)

With this change, the node is also optional:

  (dag node, node:$name, $name)

The missing node is treated as an UnsetInit, so the above is equivalent
to:

  (dag node, node:$name, ?:$name)

This syntax is useful in output patterns where we currently require the
types of variables to be repeated:

  def : Pat<(subc i32:$b, i32:$c), (SUBCCrr i32:$b, i32:$c)>;

This is preferable:

  def : Pat<(subc i32:$b, i32:$c), (SUBCCrr $b, $c)>;

llvm-svn: 177843