Improve generation of array set and get

longtemp[0] = 1 is compiled to

```
  ; allocated RSI for calculations
  mov RSI, 5  ; const index; full index=1+dims*4+index*base size
  add RSI, RBX  ; actual location
  mov DWORD [RSI], 1  ; store it!
```

but it could be 
```
  mov DWORD [RBX+5], 1  ; store it!
```