[IR] Bug fix in interaction of reqd arg and kw args.

* If only a hash is passed in, a pre-reqd arg grabs the hash
  leaving any keyword args to be initialized from defaults.

  This patch fixes handling on these two test cases

def f1(x, str: "foo", num: 4)
  [x, str, num]
end

p f1("bar" => 42)

def f2(x, **h)
  [x, h]
end

p f2({})