X87: ReceiverCheckMode needs to be utilized further.

port r24704.

original commit message:

  ReceiverCheckMode needs to be utilized further.

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

BUG=
R=weiliang.lin@intel.com

Review URL: https://codereview.chromium.org/664973002

Patch from Chunyang Dai <chunyang.dai@intel.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24714 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/x87/code-stubs-x87.cc

@@ -382,8 +382,6 @@ void LoadIndexedStringStub::Generate(MacroAssembler* masm) {
   Register result = eax;
   DCHECK(!result.is(scratch));
 
-  // TODO(mvstanton): the generator doesn't need to verify that
-  // receiver is a string map, that is done outside the handler.
   StringCharAtGenerator char_at_generator(receiver, index, scratch, result,
                                           &miss,  // When not a string.
                                           &miss,  // When not a number.
@@ -2848,8 +2846,9 @@ void SubStringStub::Generate(MacroAssembler* masm) {
   // ebx: instance type
   // ecx: sub string length (smi)
   // edx: from index (smi)
-  StringCharAtGenerator generator(
-      eax, edx, ecx, eax, &runtime, &runtime, &runtime, STRING_INDEX_IS_NUMBER);
+  StringCharAtGenerator generator(eax, edx, ecx, eax, &runtime, &runtime,
+                                  &runtime, STRING_INDEX_IS_NUMBER,
+                                  RECEIVER_IS_STRING);
   generator.GenerateFast(masm);
   __ ret(3 * kPointerSize);
   generator.SkipSlow(masm, &runtime);