Optimize away call data refs in JIT-ed method calls

According to ko1, `cd->cc != cc` was for GC.compact guard.
As we pin cc by rb_gc_mark(), we don't need the check.

```
$ benchmark-driver benchmark.yml -v --rbenv 'before --jit;after --jit' --repeat-count=12 --output=all
before --jit: ruby 2.8.0dev (2020-03-11T05:36:48Z master da6948753e) +JIT [x86_64-linux]
after --jit: ruby 2.8.0dev (2020-03-11T06:26:34Z master 36b20b8b4a) +JIT [x86_64-linux]
Calculating -------------------------------------
                                 before --jit           after --jit
Optcarrot Lan_Master.nes    74.03480698689405     71.63404803273507 fps
                            74.15085286586992     73.43923328104295
                            75.51738277744781     75.75465268365384
                            76.24922600109410     76.74071607861318
                            76.45513422802325     77.47521029238116
                            76.86617230739330     78.14759496269018
                            77.71509137131933     79.14051571125866
                            77.72839157096146     79.35884822673313
                            78.25218904561633     79.92538876408051
                            78.72521071333249     79.98075556706726
                            78.79950460165091     80.51747831497875
                            79.43884960720381     80.97973166525254
```

1 files changed, 2 insertions, 1 deletions

diff --git a/iseq.c b/iseq.c
index 0697fef53b..b6d463526d 100644
--- a/iseq.c
+++ b/iseq.c
@@ -364,7 +364,8 @@ rb_iseq_mark(const rb_iseq_t *iseq)
             for (unsigned int i=0; i<body->ci_size; i++) {
                 const struct rb_callcache *cc = cc_entries[i];
                 if (cc != NULL) {
-                    rb_gc_mark((VALUE)cc); // pindown
+                    // Pin cc against GC.compact as the the address may be written in JIT-ed code.
+                    rb_gc_mark((VALUE)cc);
                 }
             }
         }