Fix false x64 argument index assertion

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 5849cf4ef910d6573c75dd7b4858a964e75192b9..e4cd5cc26eec1612b3c110835cbcce1be8057ef5 100644
--- a/src/x64/lithium-codegen-x64.cc
+++ b/src/x64/lithium-codegen-x64.cc
@@ -2918,9 +2918,14 @@ void LCodeGen::DoAccessArgumentsAt(LAccessArgumentsAt* instr) {
       instr->index()->IsConstantOperand()) {
     int32_t const_index = ToInteger32(LConstantOperand::cast(instr->index()));
     int32_t const_length = ToInteger32(LConstantOperand::cast(instr->length()));
-    StackArgumentsAccessor args(arguments, const_length,
-                                ARGUMENTS_DONT_CONTAIN_RECEIVER);
-    __ movp(result, args.GetArgumentOperand(const_index));
+    if (const_index < const_length) {
+      StackArgumentsAccessor args(arguments, const_length,
+                                  ARGUMENTS_DONT_CONTAIN_RECEIVER);
+      __ movp(result, args.GetArgumentOperand(const_index));
+    } else {
+      // This code should never be executed; just stop here.
+      __ int3();
+    }
   } else {
     Register length = ToRegister(instr->length());
     // There are two words between the frame pointer and the last argument.