When minifying, a susprisingly large amount of bytes in the
resulting .js file are caused by the `prototype`s in:

    C.prototype.f = function(...) { ... };
    C.prototype.g = function(...) { ... };

We can get rid of them by assigning `C.prototype` once to a
temporary variable, then reusing it many times:

    $p = C.prototype;
    $p.f = function(...) { ... };
    $p.f = function(...) { ... };

This commit implements that strategy when the `minify` config
is on.

* Merge `isInterface` and `isJSType` as a single parameter `kind`
  that is an integer.
* Use the first element of `ancestors` instead of independently
  passing the `internalName` (this is safe because ES 2015
  guarantees the order of `getOwnPropertyNames`).
* Really remove the `parentData` parameter when reachability
  analysis says it is not accessed.

This removes the largest source of uncompressible and non-removable
occurrences of `$classData` identifiers.

It does increase the size of the GCC output in the `LibrarySizeTest`,
but it does not translate to the `reversi` checksizes, so it is
probably a small codebase artifact.

During the optimizer, when emitting a `VarDef(x, ..., rhs)`, we
try to inline it if it has been used exactly once. In order to do
that, we look at the `body` in which it will be available, and
replace its only occurrence if it occurs in the first evaluation
context following only pure subexpressions.

See the long comment in the code for more details.