Fix the implementation of TypeVariable.hashCode()

[Ladicek]

Previous implementation of this method was wrong in case the type variable has an implicit bound of Object. To conserve memory, TypeVariable stores the information whether the first bound is implicitly Object as the most significant bit of the hash field, which also serves as a hash code cache.

Specifically, if the type variable has an implicit Object bound, the MSB is set to 1, otherwise it is set to 0. When it's 1, the hashCode() method used to return one value when first called and a different value for subsequent calls.

This is because the beginning of hashCode() looks like this:

int hash = this.hash & HASH_MASK;
if (hash != 0) {
    return hash;
}

When the hash code has already been computed, the method returns the stored value, properly masked to always have a 0 at the MSB. When the hash code has not been computed yet, it is computed, cached and returned. The code for caching the computed value and returning it used to look like this:

return this.hash |= (hash & HASH_MASK);

That is partly wrong. The hash code is cached correctly, but then the whole value of this.hash is returned, including the non-masked MSB, which is 1 when the first bound is implicitly Object.

This commit splits the two operations (storing into cache and returning) so that the hashCode() method always returns a properly masked value:

hash &= HASH_MASK;
this.hash |= hash;
return hash;

[Ladicek]

For historical record, this bug was introduced in this commit: 9133038