Previously, in order to support a specific-arity path for Ruby
method bodies, we emitted both a variable-arity version (with
arity checks) and a specific-arity version. This doubled the
amount of bytecode emitted for a given method body, and also
doubled the code generated for any contained blocks or metaclass
bodies. This commit modifies method jit to make the variable-arity
path just call the specific-arity path when present, reducing
generated bytecode by up to 50%.