Avoid creating method objects unnecessarily when distinguishing between commands and statements.

[tompng]

It's better not to create method object unnecessarily.

Found by this bug report: https://bugs.ruby-lang.org/issues/21673
Segfaults at KERNEL_PUBLIC_METHOD.bind_call(main, command) in context.rb:636

# Step 1
irb(main):001* module Foo
irb(main):002*   refine Kernel do
irb(main):003*      def puts; end
irb(main):004*   end
irb(main):005> end
=> #<refinement:Kernel@Foo>
# Step 2: Type "puts"
irb(main):006> put/path/to/lib/irb/context.rb:636: [BUG] Segmentation fault at 0x0000000000000030
...