8242316: Shenandoah: Turn NULL-check into assert in SATB slow-path entry

Reviewed-by: zgu, shade

src/hotspot/share/gc/shenandoah/shenandoahRuntime.cpp

@@ -43,10 +43,7 @@ void ShenandoahRuntime::arraycopy_barrier_narrow_oop_entry(narrowOop* src, narro
 
 // Shenandoah pre write barrier slowpath
 JRT_LEAF(void, ShenandoahRuntime::write_ref_field_pre_entry(oopDesc* orig, JavaThread *thread))
-  if (orig == NULL) {
-    assert(false, "should be optimized out");
-    return;
-  }
+  assert(orig != NULL, "should be optimized out");
   shenandoah_assert_correct(NULL, orig);
   // store the original value that was in the field reference
   assert(ShenandoahThreadLocalData::satb_mark_queue(thread).is_active(), "Shouldn't be here otherwise");