[llvm] [TableGen][Docs] Accept "code" as a Type (PR #124902)

[Jay Foad via llvm-commits]

jayfoad wrote:

> We decided to eliminate the code type because it is identical to string. I thought I had eliminated it completely, but apparently not. Is it used in any source files?

Yes, quite a few, e.g.:
```
$ grep '^ *code ' include/llvm/Target/*.td
include/llvm/Target/TargetMacroFusion.td:  code Predicate = pred;
include/llvm/Target/TargetSchedule.td:  code Code = c;
include/llvm/Target/TargetSchedule.td:  code Predicate = pred;
include/llvm/Target/TargetSelectionDAG.td:  code XFormFunction = xformFunction;
include/llvm/Target/TargetSelectionDAG.td:  code PredicateCode = pred;
include/llvm/Target/TargetSelectionDAG.td:  code GISelPredicateCode = [{}];
include/llvm/Target/TargetSelectionDAG.td:  code ImmediateCode = [{}];
include/llvm/Target/Target.td:  code AltOrderSelect = [{}];
include/llvm/Target/Target.td:  code MCOperandPredicate;
```

https://github.com/llvm/llvm-project/pull/124902