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)>;

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177843 91177308-0d34-0410-b5e6-96231b3b80d8

docs/TableGen/LangRef.rst

@@ -286,7 +286,7 @@ given values.
 .. productionlist::
    SimpleValue: "(" `DagArg` `DagArgList` ")"
    DagArgList: `DagArg` ("," `DagArg`)*
-   DagArg: `Value` [":" `TokVarName`]
+   DagArg: `Value` [":" `TokVarName`] | `TokVarName`
 
 The initial :token:`DagArg` is called the "operator" of the dag.