Wouldn't this be better with bitfields?

[Walter Bright]

Take a look at this instruction:

https://www.scs.stanford.edu/~zyedidia/arm64/encodingindex.html#addsub_imm

And this bit of implementation:

https://github.com/dlang/dmd/pull/16554/files#diff-230d65c83f6bee1f96ea368513dbbe744372ed550c54ce6f0be171ef0848815dR38

```
/* Add/subtract (immediate)
  * ADD/ADDS/SUB/SUBS Rd,Rn,#imm{, shift}
  * https://www.scs.stanford.edu/~zyedidia/arm64/encodingindex.html#addsub_imm
  */
static uint addsub_imm(uint sf, uint op, uint S, uint sh, uint imm12, ubyte Rn, 
ubyte Rd)
{
     return (sf     << 31) |
            (op     << 30) |
            (S      << 29) |
            (0x22   << 23) |
            (sh     << 22) |
            (imm12  << 10) |
            (Rn     <<  5) |
             Rd;
}
```
That's just to initialize it. Never mind the shift/mask code to read a fields, 
or change just one of the fields. There's a large number of these functions in 
instr.d. Wouldn't it be better with:

```
struct addsub_imm { uint Rd:5, Rn:5, imm12:12, sh:1, x22:6, S:1, op:1, sf: 1; }
```

??