MIPS64: ReceiverCheckMode needs to be utilized further.

src/mips64/code-stubs-mips64.cc

Index: src/mips64/code-stubs-mips64.cc
diff --git a/src/mips64/code-stubs-mips64.cc b/src/mips64/code-stubs-mips64.cc
index f75792da7bf0fee370ed9a264415f896bed52fc3..4a867f103434967407ecc97bfb69778b8fa275e4 100644
--- a/src/mips64/code-stubs-mips64.cc
+++ b/src/mips64/code-stubs-mips64.cc
@@ -3357,8 +3357,8 @@ void SubStringStub::Generate(MacroAssembler* masm) {
   // a1: instance type
   // a2: length
   // a3: from index (untagged)
-  StringCharAtGenerator generator(
-      v0, a3, a2, v0, &runtime, &runtime, &runtime, STRING_INDEX_IS_NUMBER);
+  StringCharAtGenerator generator(v0, a3, a2, v0, &runtime, &runtime, &runtime,
+                                  STRING_INDEX_IS_NUMBER, RECEIVER_IS_STRING);
   generator.GenerateFast(masm);
   __ DropAndRet(3);
   generator.SkipSlow(masm, &runtime);