Fix type inference problem

[soutaro]

Nested block calls with type variable reports unexpected type error.

"".yield_self do       # { (String) -> X1 } -> X1
  123.yield_self do    # { (Integer) -> X2 } -> X2
    true               # bool
  end
end

Here, we expect the type propagation from bool to X2 to X1, but type checker first solves X2 to X1. And bool cannot be X1, because X1 is not a bound type variable in 123.yield_self call.

This PR is to limit generating constraints between X2 and X1 only if the hint (X1) is a free variable of self. (For generic classes.)

This was caused by #789.