8013872: G1: HeapRegionSeq::shrink_by() has invalid assert Summary: Refactored shrink_by() to only use region counts and not byte sizes Reviewed-by: johnc, tschatzl

8006088: Incompatible heap size flags accepted by VM Summary: Make processing of minimum, initial and maximum heap size more intiutive by removing previous limitations on allowed values, and make error reporting consistent. Further, fix errors in ergonomic heap sizing. Reviewed-by: johnc, jwilhelm, tamao