Consider fast path for "variable calls"

[amomchilov]

Perhaps it's possible to add a fast path in PM_NODE_CALL handling whenever the PM_CALL_NODE_FLAGS_VARIABLE_CALL flag is set, to return without checking for any of the more complex method call stuff.

https://ruby.github.io/prism/c/ast_8h.html#aa21a6c05e0dbf509b7e5b9fb61361cf2ae55caacd3eda78884aaa519ad5538a23