8335589: Fix -Wzero-as-null-pointer-constant warnings in IdealLoopTree ctor

Reviewed-by: thartmann

Author: Kim Barrett

src/hotspot/share/opto/loopnode.hpp

@@ -607,7 +607,7 @@ class IdealLoopTree : public ResourceObj {
   bool  _allow_optimizations;   // Allow loop optimizations
 
   IdealLoopTree( PhaseIdealLoop* phase, Node *head, Node *tail )
-    : _parent(0), _next(0), _child(0),
+    : _parent(nullptr), _next(nullptr), _child(nullptr),
       _head(head), _tail(tail),
       _phase(phase),
       _local_loop_unroll_limit(0), _local_loop_unroll_factor(0),