AMDGPU/SI: Add debugging subtarget feature for DS offsets

AMDGPU/SI: Add debugging subtarget feature for DS offsets

We don't have a good way to detect most situations where
DS offsets are usable on SI, so add an option to force using
them even if unsafe for debugging performance problems.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@241462 91177308-0d34-0410-b5e6-96231b3b80d8