AMDGPU: Select scratch mubuf offsets when pointer is a constant

In call sequence setups, there may not be a frame index base
and the pointer is a constant offset from the frame
pointer / scratch wave offset register.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@301230 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed