All other cost helper functions are using signed int to hold cost
while seq_cost is using unsigned int.

This fix this. bootstrap OK on x86.

OK for trunk?

2015-09-08  Jiong Wang  <jiong.wang@arm.com>

gcc/
  * rtl.h (seq_cost): Change return type from "unsigned" to "int".
  * rtlanal.c (seq_cost): Likewise.
  
-- 
Regards,
Jiong