Avoid generating opt_send with cfunc cc with JIT

only for opt_nil_p and opt_not.

While vm_method_cfunc_is is used for opt_eq too, many fast paths of it
don't call it. So if it's populated, it should generate opt_send,
regardless of cfunc or not. And again, opt_neq isn't relevant due to the
difference in operands.
So opt_nil_p and opt_not are the only variants using vm_method_cfunc_is
like they use.

```
$ benchmark-driver -v --rbenv 'before2 --jit::ruby --jit;before --jit;after --jit' benchmark/mjit_opt_cc_insns.yml --repeat-count=4
before2 --jit: ruby 2.8.0dev (2020-06-22T08:37:37Z master 3238641750) +JIT [x86_64-linux]
before --jit: ruby 2.8.0dev (2020-06-23T01:01:24Z master 9ce2066209) +JIT [x86_64-linux]
after --jit: ruby 2.8.0dev (2020-06-23T06:58:37Z master 17e9df3157) +JIT [x86_64-linux]
last_commit=Avoid generating opt_send with cfunc cc with JIT
Calculating -------------------------------------
                     before2 --jit  before --jit  after --jit
        mjit_nil?(1)       54.204M       75.536M      75.031M i/s -     40.000M times in 0.737947s 0.529548s 0.533110s
         mjit_not(1)       53.822M       70.921M      71.920M i/s -     40.000M times in 0.743195s 0.564007s 0.556171s
     mjit_eq(1, nil)        7.367M        6.496M       7.331M i/s -      8.000M times in 1.085882s 1.231470s 1.091327s

Comparison:
                     mjit_nil?(1)
        before --jit:  75536059.3 i/s
         after --jit:  75031409.4 i/s - 1.01x  slower
       before2 --jit:  54204431.6 i/s - 1.39x  slower

                      mjit_not(1)
         after --jit:  71920324.1 i/s
        before --jit:  70921063.1 i/s - 1.01x  slower
       before2 --jit:  53821697.6 i/s - 1.34x  slower

                  mjit_eq(1, nil)
       before2 --jit:   7367280.0 i/s
         after --jit:   7330527.4 i/s - 1.01x  slower
        before --jit:   6496302.8 i/s - 1.13x  slower
```

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