Clear card table for old regions during scan in G1

Particularly on large heaps the clear card table phase can take a significant amount of time in G1.

For regions that G1 scans over during remembered set scan (i.e. old gen regions), the dirty marks could be cleaned during scanning them instead of marking them as scanned if g1 only performs the initial evacuation step.

This is at least trivially always the case in young-only collections (typically large majority of collections), but also if the optional collection set candidate set is empty (which is very often the case for mixed gcs).