[amdgpu] Run SROA after loop unrolling.

Summary: - There are promotable `alloca`s after loop unrolling.

Reviewers: rampitec, arsenm

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D84252