NISTYPE takes an unsigned immediate; it should be zero extended not sign extended

```
  let imm_val : bits('m)             = sign_extend(simm);
```

should be

```
  let imm_val : bits('m)             = zero_extend(imm);
```

Spec:

> The shift amount can come from a vector register group, or a scalar x register, or **a zero-extended 5-bit immediate**. 

Also for `NITYPE`.