[runtime] Use "the hole" instead of smi 0 as sentinel for context extension.

src/full-codegen/arm64/full-codegen-arm64.cc

Index: src/full-codegen/arm64/full-codegen-arm64.cc
diff --git a/src/full-codegen/arm64/full-codegen-arm64.cc b/src/full-codegen/arm64/full-codegen-arm64.cc
index bd2c671e3137961dd7bd4b9a6c45f530a76c6171..32440bbcda38904cd3f918ec4f4835338f9ec470 100644
--- a/src/full-codegen/arm64/full-codegen-arm64.cc
+++ b/src/full-codegen/arm64/full-codegen-arm64.cc
@@ -1264,9 +1264,9 @@ void FullCodeGenerator::EmitLoadGlobalCheckExtensions(VariableProxy* proxy,
   while (s != NULL) {
     if (s->num_heap_slots() > 0) {
       if (s->calls_sloppy_eval()) {
-        // Check that extension is NULL.
+        // Check that extension is "the hole".
         __ Ldr(temp, ContextMemOperand(current, Context::EXTENSION_INDEX));
-        __ Cbnz(temp, slow);
+        __ JumpIfNotRoot(temp, Heap::kTheHoleValueRootIndex, slow);
       }
       // Load next context in chain.
       __ Ldr(next, ContextMemOperand(current, Context::PREVIOUS_INDEX));
@@ -1287,9 +1287,9 @@ void FullCodeGenerator::EmitLoadGlobalCheckExtensions(VariableProxy* proxy,
     // Terminate at native context.
     __ Ldr(temp, FieldMemOperand(next, HeapObject::kMapOffset));
     __ JumpIfRoot(temp, Heap::kNativeContextMapRootIndex, &fast);
-    // Check that extension is NULL.
+    // Check that extension is "the hole".
     __ Ldr(temp, ContextMemOperand(next, Context::EXTENSION_INDEX));
-    __ Cbnz(temp, slow);
+    __ JumpIfNotRoot(temp, Heap::kTheHoleValueRootIndex, slow);
     // Load next context in chain.
     __ Ldr(next, ContextMemOperand(next, Context::PREVIOUS_INDEX));
     __ B(&loop);
@@ -1312,18 +1312,18 @@ MemOperand FullCodeGenerator::ContextSlotOperandCheckExtensions(Variable* var,
   for (Scope* s = scope(); s != var->scope(); s = s->outer_scope()) {
     if (s->num_heap_slots() > 0) {
       if (s->calls_sloppy_eval()) {
-        // Check that extension is NULL.
+        // Check that extension is "the hole".
         __ Ldr(temp, ContextMemOperand(context, Context::EXTENSION_INDEX));
-        __ Cbnz(temp, slow);
+        __ JumpIfNotRoot(temp, Heap::kTheHoleValueRootIndex, slow);
       }
       __ Ldr(next, ContextMemOperand(context, Context::PREVIOUS_INDEX));
       // Walk the rest of the chain without clobbering cp.
       context = next;
     }
   }
-  // Check that last extension is NULL.
+  // Check that last extension is "the hole".
   __ Ldr(temp, ContextMemOperand(context, Context::EXTENSION_INDEX));
-  __ Cbnz(temp, slow);
+  __ JumpIfNotRoot(temp, Heap::kTheHoleValueRootIndex, slow);
 
   // This function is used only for loads, not stores, so it's safe to
   // return an cp-based operand (the write barrier cannot be allowed to