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

1 files changed, 1 insertions, 1 deletions

diff --git a/insns.def b/insns.def
index 018e3112ba..371361512f 100644
--- a/insns.def
+++ b/insns.def
@@ -250,7 +250,7 @@ setclassvariable
 // attr bool leaf = false; /* has rb_warning() */
 {
     vm_ensure_not_refinement_module(GET_SELF());
-    vm_setclassvariable(GET_ISEQ(), vm_get_cref(GET_EP()), GET_CFP(), id,  val, (ICVARC)ic);
+    vm_setclassvariable(GET_ISEQ(), GET_CFP(), id,  val, (ICVARC)ic);
 }
 
 /* Get constant variable id. If klass is Qnil and allow_nil is Qtrue, constants