Fix clang compilation error.

src/x64/code-stubs-x64.cc

Index: src/x64/code-stubs-x64.cc
diff --git a/src/x64/code-stubs-x64.cc b/src/x64/code-stubs-x64.cc
index a12e92eab6f6cd3f3e257b6f8ccfe61bb5ad38ae..8e6fc4496a09bdb371fecac9dbfbe09a438cb101 100644
--- a/src/x64/code-stubs-x64.cc
+++ b/src/x64/code-stubs-x64.cc
@@ -2739,14 +2739,16 @@ void InstanceofStub::Generate(MacroAssembler* masm) {
 
 void StringCharCodeAtGenerator::GenerateFast(MacroAssembler* masm) {
   // If the receiver is a smi trigger the non-string case.
-  __ JumpIfSmi(object_, receiver_not_string_);
-
-  // Fetch the instance type of the receiver into result register.
-  __ movp(result_, FieldOperand(object_, HeapObject::kMapOffset));
-  __ movzxbl(result_, FieldOperand(result_, Map::kInstanceTypeOffset));
-  // If the receiver is not a string trigger the non-string case.
-  __ testb(result_, Immediate(kIsNotStringMask));
-  __ j(not_zero, receiver_not_string_);
+  if (check_mode_ == RECEIVER_IS_UNKNOWN) {
+    __ JumpIfSmi(object_, receiver_not_string_);
+
+    // Fetch the instance type of the receiver into result register.
+    __ movp(result_, FieldOperand(object_, HeapObject::kMapOffset));
+    __ movzxbl(result_, FieldOperand(result_, Map::kInstanceTypeOffset));
+    // If the receiver is not a string trigger the non-string case.
+    __ testb(result_, Immediate(kIsNotStringMask));
+    __ j(not_zero, receiver_not_string_);
+  }
 
   // If the index is non-smi trigger the non-smi case.
   __ JumpIfNotSmi(index_, &index_not_smi_);