Make G1CMRootRegions independent of HeapRegions

G1CMRootRegions is used to store the ranges of memory that needs to be scanned through for references into the old gen heap below TAMS before marking, as roots.

Currently for storing the area to scan, it stores HeapRegions and uses its members to determine the actual range of memory that it needs to scan.

For some future improvements (e.g. JDK-8220089, but also others) this is too inflexible as the HeapRegion members change concurrently to that root region scanning.

Change the code to get passed raw MemRegions instead so that this dependency on HeapRegion members (and HeapRegion itself) goes away.

This is a split-off of JDK-8220089.