Check for young region in G1BarrierSet::invalidate instead of card-by-card check

G1BarrierSet::invalidate() checks whether the given Memregion to invalidate is in young gen by checking the corresponding young-card values.

This is very slow compared to a simple is-start-in-young-gen check as
the memregion should be either completely in young gen or not, so a single check for that is sufficient.

Also add necessary asserts to verify that.