Fuzzer Not Detecting Incorrect Allocation

It's possible for vregs to be used or defined in branch instructions, but the fuzzer doesn't seem to check for these operands.

The following doesn't pass the fuzzer:

```
block0:
0. branch(1). operands: [def v0 (fixed: p0)] // Allocation: [v0: p0]

block1:
1. operands: [use v0 (fixed: p0)] // Allocation: [v0: p0]
```

And this does:

```
block0:
0. operand: [def v0 (fixed: p0)] // Allocation: [v0: p0]
1. branch(1). operands: [use v0 (fixed: p1)] // Allocation: [v0: p9]

block1:
2. operands: [use v0 (fixed: p0)] // Allocation: [v0: p0]
```

Which is incorrect.

To reproduce this, run the tests in `src/fastalloc/tests.rs` at https://github.com/d-sonuga/regalloc2/tree/975dee0ceb56bbc6cbd21554a237babe1e388573.

To resolve this issue, one of the following could be done:

- Disallow operands in branch instructions.
- Update the fuzzer to check for these operands.