Change store buffer overflow check to not rely on the store buffer being (2*Size) aligned.

src/x64/macro-assembler-x64.cc

Index: src/x64/macro-assembler-x64.cc
diff --git a/src/x64/macro-assembler-x64.cc b/src/x64/macro-assembler-x64.cc
index b0a2603e311739dc40cd956b2af23e7346f54229..566091df4e2d48c65dc4c91b0e67f118f7ac7afc 100644
--- a/src/x64/macro-assembler-x64.cc
+++ b/src/x64/macro-assembler-x64.cc
@@ -227,15 +227,15 @@ void MacroAssembler::RememberedSetHelper(Register object,  // For debug tests.
   // Call stub on end of buffer.
   Label done;
   // Check for end of buffer.
-  testp(scratch, Immediate(StoreBuffer::kStoreBufferOverflowBit));
+  testp(scratch, Immediate(StoreBuffer::kStoreBufferMask));
   if (and_then == kReturnAtEnd) {
     Label buffer_overflowed;
-    j(not_equal, &buffer_overflowed, Label::kNear);
+    j(equal, &buffer_overflowed, Label::kNear);
     ret(0);
     bind(&buffer_overflowed);
   } else {
     DCHECK(and_then == kFallThroughAtEnd);
-    j(equal, &done, Label::kNear);
+    j(not_equal, &done, Label::kNear);
   }
   StoreBufferOverflowStub store_buffer_overflow(isolate(), save_fp);
   CallStub(&store_buffer_overflow);