[IR] Discard additional "_" args in a block.

* Lets me run all MRI tests to completion with just a LinearizeCFG
  pass.

* Before this fix, the following snippet would have crashed with
  these args: -X-CIR -Xir.passes=LinearizeCFG

def foo
 yield [1,2,3,4]
end

foo { |a,_,_,_| p a }