R600/SI: Use DS offsets for constant addresses

Use 0 as the base address for a constant address, so if
we have a constant address we can save moves and form
read2/write2s.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219698 91177308-0d34-0410-b5e6-96231b3b80d8