ZGC: Simplify ZVerify

ZVerify has a few oddities:

1)
void ZVerify::before_zoperation() {
  // Verify strong roots
  ZStatTimerDisable disable;
  roots(false /* verify_strong */, false /* verify_weaks */);
}

The comment and the parameter names don't match. This is caued by a name clash between the layers:

roots_strong(bool verify_fixed
roots(bool verify_strong
roots_and_objects(bool verify_strong

2) Both usages of roots_and_objects pass true as the verify_strong argument:

void ZVerify::after_mark() {
  // Verify all strong roots and strong references
  ZStatTimerDisable disable;
  roots_and_objects(true /* verify_strong */, false /* verify_weaks */);
}

void ZVerify::after_weak_processing() {
  // Verify all roots and all references
  ZStatTimerDisable disable;
  roots_and_objects(true /* verify_strong */, true /* verify_weaks */);
}

The proposal is to remove the middle layers and be more explicit at the call sites.