I find myself doing this alot
```
for (unsigned varIndex = rel.getVarKindOffset(VarKind::Domain);
varIndex < rel.getVarKindEnd(VarKind::Domain); ++varIndex) {
...
}
```
Adding this convenience method so I can instead do
```
for (unsigned varIndex : rel.iterVarKind(VarKind::Domain)) {
...
}
```
---------
Co-authored-by: Jeremy Kun <j2kun@users.noreply.github.com>
97dee32445