Break the optimiser into separate functions.

The optimiser has now exceeded even my (high!) tolerance for long
functions. Previously it was structured as:

```rust
match inst {
  Inst::A => {
    ...
  }
  Inst::B => {
    ...
  }
}
```

where the `...`s could be arbitrarily long.

This commit -- which has no functional change, despite its size! --
changes the structure to:

```rust
match inst {
  Inst::A => self.opt_a(...),
  Inst::B => self.opt_b(...),
}
```

So now each instruction has its "own" function. `opt_binop` is still
fearsomely large, but this commit moves us in a more sustainable
direction.