R600/SI: Implement areMemAccessesTriviallyDisjoint

This partially makes up for not having address spaces
used for alias analysis in some simple cases.

This is not yet enabled by default so shouldn't change anything yet.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222286 91177308-0d34-0410-b5e6-96231b3b80d8