PSScavenge::should_attempt_scavenge contains some heuristics to decide whether a young-gc should be upgraded to full-gc, e.g. old-gen has low free-space, which might results into young-gc failure.

One can move such heuristics to the caller of PSScavenge::invoke, instead. This can simplify the logic in `PSScavenge::invoke` so that when it's called, it always runs to completion (always triggers a young-gc), no early-return.