8345647: Fix recent NULL usage backsliding in Shenandoah

Reviewed-by: shade, jwaters, mli

Author: Sonia Zaldana Calles

src/hotspot/cpu/ppc/gc/shenandoah/shenandoahBarrierSetAssembler_ppc.cpp

@@ -624,7 +624,7 @@ void ShenandoahBarrierSetAssembler::store_at(MacroAssembler *masm, DecoratorSet
                                 tmp1, tmp2, tmp3,
                                 preservation_level);
 
-  // No need for post barrier if storing NULL
+  // No need for post barrier if storing null
   if (ShenandoahCardBarrier && is_reference_type(type) && val != noreg) {
     store_check(masm, base, ind_or_offs, tmp1);
   }

src/hotspot/share/gc/shenandoah/c2/shenandoahBarrierSetC2.cpp

@@ -458,7 +458,7 @@ void ShenandoahBarrierSetC2::post_barrier(GraphKit* kit,
 
   // No store check needed if we're storing a null.
   if (val != nullptr && val->is_Con()) {
-    // must be either an oop or NULL
+    // must be either an oop or null
     const Type* t = val->bottom_type();
     if (t == TypePtr::NULL_PTR || t == Type::TOP)
       return;