for opt_* insns.

opt_eq handles rb_obj_equal inside opt_eq, and all other cfunc is
handled by opt_send_without_block. Therefore we can't decide which insn
should be generated by checking whether it's cfunc cc or not.

```
$ benchmark-driver -v --rbenv 'before --jit;after --jit' benchmark/mjit_opt_cc_insns.yml --repeat-count=4
before --jit: ruby 2.8.0dev (2020-06-26T05:21:43Z master 9dbc2294a6) +JIT [x86_64-linux]
after --jit: ruby 2.8.0dev (2020-06-26T06:30:18Z master 75cece1b0b) +JIT [x86_64-linux]
last_commit=Decide JIT-ed insn based on cached cfunc
Calculating -------------------------------------
                     before --jit  after --jit
        mjit_nil?(1)      73.878M      74.021M i/s -     40.000M times in 0.541432s 0.540391s
         mjit_not(1)      72.635M      74.601M i/s -     40.000M times in 0.550702s 0.536187s
     mjit_eq(1, nil)       7.331M       7.445M i/s -      8.000M times in 1.091211s 1.074596s
     mjit_eq(nil, 1)      49.450M      64.711M i/s -      8.000M times in 0.161781s 0.123627s

Comparison:
                     mjit_nil?(1)
         after --jit:  74020528.4 i/s
        before --jit:  73878185.9 i/s - 1.00x  slower

                      mjit_not(1)
         after --jit:  74600882.0 i/s
        before --jit:  72634507.6 i/s - 1.03x  slower

                  mjit_eq(1, nil)
         after --jit:   7444657.4 i/s
        before --jit:   7331304.3 i/s - 1.02x  slower

                  mjit_eq(nil, 1)
         after --jit:  64710790.6 i/s
        before --jit:  49449507.4 i/s - 1.31x  slower
```

because opt_nil/opt_not/opt_eq populates cc even when it doesn't
fallback to opt_send_without_block because of vm_method_cfunc_is.

```
$ benchmark-driver -v --rbenv 'before --jit;after --jit' benchmark/mjit_opt_cc_insns.yml --repeat-count=4
before --jit: ruby 2.8.0dev (2020-06-22T08:11:24Z master d231b8f95b) +JIT [x86_64-linux]
after --jit: ruby 2.8.0dev (2020-06-22T08:53:27Z master e1125879ed) +JIT [x86_64-linux]
last_commit=Compile opt_send for opt_* only when cc has ISeq
Calculating -------------------------------------
                     before --jit  after --jit
        mjit_nil?(1)      54.106M      73.693M i/s -     40.000M times in 0.739288s 0.542795s
         mjit_not(1)      53.398M      74.477M i/s -     40.000M times in 0.749090s 0.537075s
     mjit_eq(1, nil)       7.427M       6.497M i/s -      8.000M times in 1.077136s 1.231326s

Comparison:
                     mjit_nil?(1)
         after --jit:  73692594.3 i/s
        before --jit:  54106108.4 i/s - 1.36x  slower

                      mjit_not(1)
         after --jit:  74477487.9 i/s
        before --jit:  53398125.0 i/s - 1.39x  slower

                  mjit_eq(1, nil)
        before --jit:   7427105.9 i/s
         after --jit:   6497063.0 i/s - 1.14x  slower
```

Actually opt_eq becomes slower by this. Maybe it's indeed using
opt_send_without_block, but I'll approach that one in another commit.