Correct the pre-increment load latencies in the PPC A2 itinerary

Pre-increment loads are microcoded on the A2, and the address increment occurs
only after the load completes. As a result, the latency of the GPR address
update is an additional 2 cycles on top of the load latency.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191156 91177308-0d34-0410-b5e6-96231b3b80d8