[TableGen][Docs] Accept "code" as a Type

Previously the Type production did not include "code", which was only
accepted in one place in the grammar:

   BodyItem: (`Type` | "code") `TokIdentifier` ["=" `Value`] ";"

However the parser implementation accepts "code" as a Type with only one
place where it is *not* allowed, corresponding to this production:

   SimpleValue9: `BangOperator` ["<" `Type` ">"] "(" `ValueListNE` ")"

This patch changes the production for Type to include "code", thereby
fixing most occurrences of Type in the grammar, and documents the
restriction for BangOperator Types in the text instead of codifying it
in the grammar.

Author: jayfoad

llvm/docs/TableGen/ProgRef.rst

@@ -268,7 +268,7 @@ high-level types (e.g., ``dag``). This flexibility allows you to describe a
 wide range of records conveniently and compactly.
 
 .. productionlist::
-   Type: "bit" | "int" | "string" | "dag"
+   Type: "bit" | "int" | "string" | "dag" | "code"
        :| "bits" "<" `TokInteger` ">"
        :| "list" "<" `Type` ">"
        :| `ClassID`
@@ -285,6 +285,10 @@ wide range of records conveniently and compactly.
     The ``string`` type represents an ordered sequence of characters of arbitrary
     length.
 
+``code``
+    The keyword ``code`` is an alias for ``string`` which may be used to
+    indicate string values that are code.
+
 ``bits<``\ *n*\ ``>``
     The ``bits`` type is a fixed-sized integer of arbitrary length *n* that
     is treated as separate bits. These bits can be accessed individually.
@@ -498,6 +502,8 @@ arguments, producing a value for that bang operator. The ``!cond`` operator
 takes a list of pairs of arguments separated by colons. See `Appendix A:
 Bang Operators`_ for a description of each bang operator.
 
+The `Type` is only accepted for certain bang operators, and must not be
+``code``.
 
 Suffixed values
 ---------------
@@ -670,16 +676,15 @@ arguments.
 
 .. productionlist::
    Body: ";" | "{" `BodyItem`* "}"
-   BodyItem: (`Type` | "code") `TokIdentifier` ["=" `Value`] ";"
+   BodyItem: `Type` `TokIdentifier` ["=" `Value`] ";"
            :| "let" `TokIdentifier` ["{" `RangeList` "}"] "=" `Value` ";"
            :| "defvar" `TokIdentifier` "=" `Value` ";"
            :| `Assert`
 
 A field definition in the body specifies a field to be included in the class
 or record. If no initial value is specified, then the field's value is
 uninitialized. The type must be specified; TableGen will not infer it from
-the value. The keyword ``code`` may be used to emphasize that the field
-has a string value that is code.
+the value.
 
 The ``let`` form is used to reset a field to a new value. This can be done
 for fields defined directly in the body or fields inherited from parent