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

src/x64/macro-assembler-x64.h

Index: src/x64/macro-assembler-x64.h
diff --git a/src/x64/macro-assembler-x64.h b/src/x64/macro-assembler-x64.h
index 6408e6d6f76e25ad67802797d277c25fdab93442..dbf46782a4dff046b88c6a055cc54aad9fae6f07 100644
--- a/src/x64/macro-assembler-x64.h
+++ b/src/x64/macro-assembler-x64.h
@@ -153,6 +153,12 @@ class MacroAssembler: public Assembler {
     CompareRoot(with, index);
     j(equal, if_equal, if_equal_distance);
   }
+  void JumpIfRoot(const Operand& with, Heap::RootListIndex index,
+                  Label* if_equal,
+                  Label::Distance if_equal_distance = Label::kNear) {
+    CompareRoot(with, index);
+    j(equal, if_equal, if_equal_distance);
+  }
 
   // Compare the object in a register to a value and jump if they are not equal.
   void JumpIfNotRoot(Register with, Heap::RootListIndex index,
@@ -161,6 +167,12 @@ class MacroAssembler: public Assembler {
     CompareRoot(with, index);
     j(not_equal, if_not_equal, if_not_equal_distance);
   }
+  void JumpIfNotRoot(const Operand& with, Heap::RootListIndex index,
+                     Label* if_not_equal,
+                     Label::Distance if_not_equal_distance = Label::kNear) {
+    CompareRoot(with, index);
+    j(not_equal, if_not_equal, if_not_equal_distance);
+  }
 
   // These functions do not arrange the registers in any particular order so
   // they are not useful for calls that can cause a GC.  The caller can