Cache access to reg_cfp->self on JIT
```
$ benchmark-driver -v --rbenv 'before --jit;after --jit' --repeat-count=12 --alternate --output=all benchmark.yml
before --jit: ruby 3.0.0dev (2020-11-27T06:41:15Z master 8ce1711c25) +JIT [x86_64-linux]
after --jit: ruby 3.0.0dev (2020-11-27T08:36:02Z master 2c592126b9) +JIT [x86_64-linux]
last_commit=Cache access to reg_cfp->self on JIT
Calculating -------------------------------------
before --jit after --jit
Optcarrot Lan_Master.nes 82.40522392468650 82.66023870551237 fps
82.67998539899482 83.08660305312587
85.51280693947453 87.09311989553235
86.32925337181406 87.16115255191410
87.35617494926235 87.30699391518075
87.91865339426212 88.47590342996875
88.11573661006648 88.64778616696353
88.16060826662158 88.67015079203991
88.21639244865058 89.19630739497482
88.47241577897603 89.23443637947730
89.37087287229809 89.57052723997015
89.46969964699964 89.97803363889025
```
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