Consolidation: ExpandHeap_lock should be used uniformly.

See 6316605 for some background. We should delete ExpandHeap_lock and
redirect relevant asserts to Heap_lock.
Not quite; ExpandHeap_lock does have an essential role
to play in ParallelScavengeHeap; see the comments section.
Based on this, the RFE's Synopsys has been changed to reflect
its new role.