[Power9] Add a specific heuristic to schedule the addi before the load

 When we are scheduling the load and addi, if all other heuristic didn't take effect,
 we will try to schedule the addi before the load, to hide the latency, and avoid the
 true dependency added by RA. And this only take effects for Power9.

Differential Revision: https://reviews.llvm.org/D61930

llvm-svn: 361600