[AMDGPU] For OS type AMDPAL, fixed scratch on compute shader

Summary:
For OS type AMDPAL, the scratch descriptor is loaded from offset 0 of
the GIT, whose 32 bit pointer is in s0 (s8 for gfx9 merged shaders).

This commit fixes that to use offset 0x10 instead of offset 0 for a
compute shader, per the PAL ABI spec.

V2: Ensure s0 (s8 for gfx9 merged shader) is marked live-in when loading
scratch descriptor from GIT.

Reviewers: kzhuravl, nhaehnle, timcorringham

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

Differential Revision: https://reviews.llvm.org/D44468

Change-Id: I93dffa647758e37f613bb5e0dfca840d82e6d26f
llvm-svn: 329690