Refactor setclassvariable (#5143)

We only need the cref when we have a cache miss so don't look it up until we
need it. This likely speeds up class variable writes in the interpreter but
also simplifies the jit code.

Before

```
Warming up --------------------------------------
        write a cvar   192.280k i/100ms
Calculating -------------------------------------
        write a cvar      1.915M (± 3.5%) i/s -      9.614M in   5.026694s
```

After

```
Warming up --------------------------------------
        write a cvar   216.308k i/100ms
Calculating -------------------------------------
        write a cvar      2.140M (± 3.1%) i/s -     10.815M in   5.058079s
```

Followup to ruby/ruby#5137

0 files changed, 0 insertions, 0 deletions