8164954: split_if creates empty phi and region nodes

Summary: Don't split if all edges will be moved to new phi
Reviewed-by: kvn

src/share/vm/opto/ifnode.cpp

@@ -234,6 +234,13 @@ static Node* split_if(IfNode *iff, PhaseIterGVN *igvn) {
       predicate_proj = proj;
     }
   }
+
+  // If all the defs of the phi are the same constant, we already have the desired end state.
+  // Skip the split that would create empty phi and region nodes.
+  if((r->req() - req_c) == 1) {
+    return NULL;
+  }
+
   Node* predicate_c = NULL;
   Node* predicate_x = NULL;
   bool counted_loop = r->is_CountedLoop();