[AMDGPU] Fix incorrect arch assert while setting up FlatScratchInit

Reviewers: arsenm, foad, rampitec, scott.linder

Reviewed By: arsenm

Subscribers: kzhuravl, jvesely, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye, hiraditya, kerbowa, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D84391