[OpenCL] Fix `mix` builtin overloads

`mix` is subtly different from `clamp`: in the overloads where the
last argument is a scalar, the second argument should be a gentype for
`mix`.

As scalars can be implicitly converted to vectors, this cannot be
caught in the Sema test.  Hence adding a CodeGen test, where we can
verify the types using the mangled name.