Add infrastructure to keep track of references to prototypes.

src/x64/lithium-codegen-x64.cc

Index: src/x64/lithium-codegen-x64.cc
diff --git a/src/x64/lithium-codegen-x64.cc b/src/x64/lithium-codegen-x64.cc
index 0aa10e3d455a40e08b198366a2fbdb53feb92b37..10f2bb8cdd27a7dbeb80c86f549e83ccac6bc0ae 100644
--- a/src/x64/lithium-codegen-x64.cc
+++ b/src/x64/lithium-codegen-x64.cc
@@ -3280,11 +3280,9 @@ Operand LCodeGen::BuildFastArrayOperand(
     return Operand(elements_pointer_reg,
                    (constant_value << shift_size) + offset);
   } else {
-    // Take the tag bit into account while computing the shift size.
-    if (key_representation.IsSmi() && (shift_size >= 1)) {
-      DCHECK(SmiValuesAre31Bits());
-      shift_size -= kSmiTagSize;
-    }
+    // Guaranteed by ArrayInstructionInterface::KeyedAccessIndexRequirement().
+    DCHECK(key_representation.IsInteger32());
+
     ScaleFactor scale_factor = static_cast<ScaleFactor>(shift_size);
     return Operand(elements_pointer_reg,
                    ToRegister(key),