Fix BNF nits in TableGen language reference.

[simon_tatham]

Summary

In the course of writing an experimental ANTLR grammar based on this
document, I found three errors in the documented BNF:

SimpleValues of dag type are allowed to have no operands at all after
the initial DagArg specifying the operator. For example, the value
(outs) is extremely common in backends; an example in the test suite
is test/TableGen/AsmVariant.td line 30. But the BNF doesn't allow
DagArgList to expand to the empty string (it must contain at least one
DagArg), and therefore the DagArgList specifying the operands in the
dag-shaped production for SimpleValue should be optional.

In the production for BodyItem with a 'let' and an optional RangeList,
the RangeList should have braces around it if it's present, matching
code such as "let E{7-0} = ..." on test/TableGen/BitsInit.td line 42.
Those braces aren't included in the RangeList nonterminal itself, so
instead they need to be part of the optional segment of the BodyItem
production.

Finally, the identifier after 'defm' should be optional. Again, this
is very common in the real back end .td files; an example in the test
suite is in test/TableGen/defmclass.td line 49.

[rengolin]

Adding TableGen's code owner.

[nhaehnle]

Thanks, LGTM.

docs/TableGen/LangRef.rst

 specific element type, otherwise the element type will be deduced from the
 given values.
 
 .. The initial `DagArg` of the dag must start with an identifier or
    !cast, but this is more of an implementation detail and so for now just
    leave it out.
 
 .. productionlist::
-   SimpleValue: "(" `DagArg` `DagArgList` ")"
+   SimpleValue: "(" `DagArg` [`DagArgList`] ")"
    DagArgList: `DagArg` ("," `DagArg`)*
    DagArg: `Value` [":" `TokVarName`] | `TokVarName`
 
 The initial :token:`DagArg` is called the "operator" of the dag.
 
 .. productionlist::
    SimpleValue: `BangOperator` ["<" `Type` ">"] "(" `ValueListNE` ")"
 

 .. put this somewhere else
 
 It is after parsing the base class list that the "let stack" is applied.
 
 .. productionlist::
    Body: ";" | "{" BodyList "}"
    BodyList: BodyItem*
    BodyItem: `Declaration` ";"
-           :| "let" `TokIdentifier` [`RangeList`] "=" `Value` ";"
+           :| "let" `TokIdentifier` [ "{" `RangeList` "}" ] "=" `Value` ";"
 
 The ``let`` form allows overriding the value of an inherited field.
 
 ``def``
 -------
 
 .. TODO::
    There can be pastes in the names here, like ``#NAME#``. Look into that

 
 Special handling occurs if this ``def`` appears inside a ``multiclass`` or
 a ``foreach``.
 
 ``defm``
 --------
 
 .. productionlist::
-   Defm: "defm" `TokIdentifier` ":" `BaseClassListNE` ";"
+   Defm: "defm" [`TokIdentifier`] ":" `BaseClassListNE` ";"
 
 Note that in the :token:`BaseClassList`, all of the ``multiclass``'s must
 precede any ``class``'s that appear.
 
 ``defset``
 ----------
 .. productionlist::
    Defset: "defset" `Type` `TokIdentifier` "=" "{" `Object`* "}"