[PowerPC] P9 Scheduling Model: dispatching rule fixes

This is to address some of the problems in existing P9 resource modeling,
especially about the dispatching rules.

Instead of using a hypothetical DISPATCHER , we try to use the number of
actual dispatch slots, and define SchedWriteRes to model dispatch rules,
then update instruction classes according to dispatch rules.

All the dispatch rules and instruction classes update are made according
to POWER9 User Manual.

Differential Revision: https://reviews.llvm.org/D61873

llvm-svn: 362509