[PowerPC] Implement the areMemAccessesTriviallyDisjoint hook

[PowerPC] Implement the areMemAccessesTriviallyDisjoint hook
After implemented this hook, we will model the memory dependency in the scheduling dependency graph more precise,
and will have more opportunity to reorder the load/stores, as they didn't have the dependency at some condition

Differential Revision: https://reviews.llvm.org/D63804

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@364886 91177308-0d34-0410-b5e6-96231b3b80d8