Fix panic on check_var_ref checker

[taichi-ishitani]

fix #1127
fix #1128

[taichi-ishitani]

@nananapo

I think this PR can also be the fix for #1128.
Can you check if this PR acually resolves #1128?

[nananapo]

OK

#1127

Error:
  × veryl check failed

Error: invalid_assignment (https://doc.veryl-lang.org/book/07_appendix/02_semantic_error.html#invalid_assignment)

  × a can't be assigned because it is instance
    ╭─[/home/kanataso/Documents/test/a.veryl:14:9]
 13 │     always_comb {
 14 │         a = b;
    ·         ┬
    ·         ╰── Error location
 15 │     }
    ╰────
  help: remove the assignment

Error: invalid_factor (https://doc.veryl-lang.org/book/07_appendix/02_semantic_error.html#invalid_factor)

  × b of kind "port" cannot be used as a factor in an expression
    ╭─[/home/kanataso/Documents/test/a.veryl:14:13]
 13 │     always_comb {
 14 │         a = b;
    ·             ┬
    ·             ╰── Error location
 15 │     }
    ╰────
  help: remove port from expression

#1128

Error:
  × veryl check failed

Error: invalid_factor (https://doc.veryl-lang.org/book/07_appendix/02_semantic_error.html#invalid_factor)

  × a of kind "port" cannot be used as a factor in an expression
    ╭─[/home/kanataso/Documents/test/a.veryl:24:28]
 23 │     }
 24 │     let _b: logic = myfunc(a);
    ·                            ┬
    ·                            ╰── Error location
 25 │ }
    ╰────
  help: remove port from expression