to? should give warnings if the operation is redundant

[Chuck]

The expression a to? B is a no-op when a is already of type B or B?. There are already warnings for a to B when that is redundant, so this is missing for to?.

Should get two warnings below; currently get none.

class A

	def main is shared
		a = A()
		b = a to? A  # .warning.
		c = a to ?
		d = c to? A  # .warning.
		CobraCore.noOp(a, b, c, d)