6684385: Loop unswitching crashes without LoopNode

Summary: Without LoopNode, exit early from loop unswitching and partial peeling
Reviewed-by: kvn, never, sgoldman

src/share/vm/opto/loopUnswitch.cpp

@@ -51,6 +51,9 @@ bool IdealLoopTree::policy_unswitching( PhaseIdealLoop *phase ) const {
   if( !LoopUnswitching ) {
     return false;
   }
+  if (!_head->is_Loop()) {
+    return false;
+  }
   uint nodes_left = MaxNodeLimit - phase->C->unique();
   if (2 * _body.size() > nodes_left) {
     return false; // Too speculative if running low on nodes.

src/share/vm/opto/loopopts.cpp

@@ -2257,6 +2257,9 @@ bool PhaseIdealLoop::is_valid_clone_loop_form( IdealLoopTree *loop, Node_List& p
 //
 bool PhaseIdealLoop::partial_peel( IdealLoopTree *loop, Node_List &old_new ) {
 
+  if (!loop->_head->is_Loop()) {
+    return false;  }
+
   LoopNode *head  = loop->_head->as_Loop();
 
   if (head->is_partial_peel_loop() || head->partial_peel_has_failed()) {