We have a bunch of ugly hacks in GC code paths where we do stuff like:
if (UseCompactObjectHeaders) {
 // fetch object Klass* or size in a safe way...
} else {
  // fetch object Klass* or size in the traditional way
}

This happens in various places in similar ways. I propose to add relevant methods to oopDesc, and make all GC code use those new methods instead.

This helps streamline and clean up a bunch of GC code.