AMDGPU/SI: Query AA, if available, in areMemAccessesTriviallyDisjoint()

Summary:
The SILoadStoreOptimizer will need to use AliasAnalysis here in order to
move it before scheduling.

Reviewers: arsenm

Subscribers: arsenm, llvm-commits, kzhuravl

Differential Revision: https://reviews.llvm.org/D23813

llvm-svn: 279963