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.

Start by running the tests in src/fastalloc/tests.rs at the referenced commit and inspect how branch instruction operands are represented and checked. Compare the two provided examples, then determine whether branch operands should be disallowed or included in fuzzer validation. Done means the incorrect allocation case is rejected or the unsupported operands are prevented.

Written by the indexing model from the issue text.