MIPS64: ReceiverCheckMode needs to be utilized further.

Port r24704 (f43a27ee)

Original commit message:
The parameter wasn't being passed appropriately, and there was an extra
opportunity to use mode RECEIVER_IS_STRING in SubStringStub.

BUG=
R=paul.lind@imgtec.com

Review URL: https://codereview.chromium.org/645823003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24707 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

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);