[mlir][scf] Add an IndexSwitchOp

The `scf.index_switch` is a control-flow operation that branches to one of the
given regions based on the values of the argument and the cases. The
argument is always of type `index`.

Example:

```mlir
%0 = scf.index_switch %arg0 -> i32
case 2 {
  %1 = arith.constant 10 : i32
  scf.yield %1 : i32
}
case 5 {
  %2 = arith.constant 20 : i32
  scf.yield %2 : i32
}
default {
  %3 = arith.constant 30 : i32
  scf.yield %3 : i32
}
```

Reviewed By: jpienaar

Differential Revision: https://reviews.llvm.org/D136003