[Prism] Fix cvar or assignment instructions

The instructions for `PM_CLASS_VARIABLE_OR_WRITE_NODE` were incorrect as
they were missing a `putnil`, a `defined`, and a `branchunless`.

I verified this is fixed via the instructions and running the following:
`RUBY_ISEQ_DUMP_DEBUG=prism make test/csv/interface/test_read_write.rb`.

These new instructions can't go in the defined function because
`defined?(@@fop ||= 1)` should return "assignment" not "class variable".

Instructions before:

```
"********* Ruby *************"
== disasm: #<ISeq:<compiled>@<compiled>:1 (1,0)-(30,11)>
0000 putnil                                                           (  30)[Li]
0001 defined                                class variable, :@@foo, true
0005 branchunless                           14
0007 getclassvariable                       :@@foo, <is:0>
0010 dup
0011 branchif                               20
0013 pop
0014 putobject                              1
0016 dup
0017 setclassvariable                       :@@foo, <is:0>
0020 leave

"********* PRISM *************"
== disasm: #<ISeq:<compiled>@<compiled>:29 (29,0)-(29,11)>
0000 getclassvariable                       :@@foo, <is:0>            (  29)[Li]
0003 dup
0004 branchif                               13
0006 pop
0007 putobject                              1
0009 dup
0010 setclassvariable                       :@@foo, <is:0>
0013 leave
```

Instructions after:

```
"********* Ruby *************"
== disasm: #<ISeq:<compiled>@<compiled>:1 (1,0)-(30,11)>
0000 putnil                                                           (  30)[Li]
0001 defined                                class variable, :@@foo, true
0005 branchunless                           14
0007 getclassvariable                       :@@foo, <is:0>
0010 dup
0011 branchif                               20
0013 pop
0014 putobject                              1
0016 dup
0017 setclassvariable                       :@@foo, <is:0>
0020 leave

"********* PRISM *************"
== disasm: #<ISeq:<compiled>@<compiled>:29 (29,0)-(29,11)>
0000 putnil                                                           (  29)[Li]
0001 defined                                class variable, :@@foo, true
0005 branchunless                           14
0007 getclassvariable                       :@@foo, <is:0>
0010 dup
0011 branchif                               20
0013 pop
0014 putobject                              1
0016 dup
0017 setclassvariable                       :@@foo, <is:0>
0020 leave
```

Fixes ruby/prism#2064

0 files changed, 0 insertions, 0 deletions