6944166: G1: explicit GCs are not always handled correctly

G1 was not handling explicit GCs correctly in many ways. It does now. See the CR for the list of improvements contained in this changeset.

Reviewed-by: iveresov, ysr, johnc

hotspot/src/share/vm/gc_interface/gcCause.cpp

@@ -78,6 +78,9 @@ const char* GCCause::to_string(GCCause::Cause cause) {
     case _old_generation_too_full_to_scavenge:
       return "Old Generation Too Full To Scavenge";
 
+    case _g1_inc_collection_pause:
+      return "G1 Evacuation Pause";
+
     case _last_ditch_collection:
       return "Last ditch collection";