[AMDGPU] Scratch setup fix on AMDPAL gfx9+ merge shader

Summary:
With OS type AMDPAL, the scratch descriptor is hardwired to be loaded
from offset 0 of the global information table, whose low pointer is
passed in s0. For a merge shader on gfx9+, it needs to be s8 instead, as
the hardware reserves s0-s7.

Reviewers: kzhuravl

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

Differential Revision: https://reviews.llvm.org/D42203

llvm-svn: 326088