Fix clang++ warnings on -Wtautological-constant-out-of-range-compare

BUG=v8:3730
LOG=N
R=dslomov@chromium.org

Review URL: https://codereview.chromium.org/750093003

Cr-Commit-Position: refs/heads/master@{#25579}

src/elements.h

@@ -179,7 +179,7 @@ class ElementsAccessor {
 
   // Returns a shared ElementsAccessor for the specified ElementsKind.
   static ElementsAccessor* ForKind(ElementsKind elements_kind) {
-    DCHECK(elements_kind < kElementsKindCount);
+    DCHECK(static_cast<int>(elements_kind) < kElementsKindCount);
     return elements_accessors_[elements_kind];
   }
 

src/objects.h

@@ -5739,7 +5739,7 @@ class Map: public HeapObject {
   inline bool is_prototype_map();
 
   inline void set_elements_kind(ElementsKind elements_kind) {
-    DCHECK(elements_kind < kElementsKindCount);
+    DCHECK(static_cast<int>(elements_kind) < kElementsKindCount);
     DCHECK(kElementsKindCount <= (1 << Map::ElementsKindBits::kSize));
     set_bit_field2(Map::ElementsKindBits::update(bit_field2(), elements_kind));
     DCHECK(this->elements_kind() == elements_kind);