[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

llvm-svn: 364886