[X86] Use ADD/SUB instead of INC/DEC for Silvermont

According to Intel Software Optimization Manual 
on Silvermont INC or DEC instructions require 
an additional uop to merge the flags.
As a result, a branch instruction depending 
on an INC or a DEC instruction incurs a 1 cycle penalty.

Differential Revision: http://reviews.llvm.org/D3990

llvm-svn: 210466