AMDGPU/SI: Don't reserve FLAT_SCR on non-HSA targets & without stack objects

Summary: This frees 2 scalar registers.

Reviewers: tstellarAMD

Subscribers: qcolombet, arsenm, kzhuravl, wdng, nhaehnle, yaxunl, tony-tye

Differential Revision: https://reviews.llvm.org/D27150

llvm-svn: 289261