Honestly I'd just go:
```python
   with m.Switch(instr[12:15]):
      with m.Case(0):
         m.d.comb += aluout_base.eq(instr[30] & instr[5] ? aluMinus[31:0] : aluPlus)
      with m.Case(1):
         m.d.comb += aluout_base.eq(leftshift)
      [...]
```