AMDGPU: GFX9 GS and HS shaders always have the scratch wave offset in SGPR5

Reviewers: arsenm, nhaehnle

Subscribers: kzhuravl, wdng, yaxunl, dstuttard, tpr, t-tye, llvm-commits

Differential Revision: https://reviews.llvm.org/D32645

llvm-svn: 302200