Compiler: Lower level Code.Switch

`Code.Switch` currently deal with both immediate and block/tagged values.
I feel we could simplify things a bit by adding an explicit condition on `is_int` and have `Switch` just dispatch on a variable immediate value

In other words, I means turning 
```
B1: Switch(x,A1,A2)
```

Into
```
B1: Cond(isInt x, B2, B3)
B2: Switch(x, A1)
B3: let x' = tag x; Switch (x', A2)
```

