[AMDGPU] Fix non-deterministic iteration order

Summary:
As far as I know this did not affect code generation, but it did affect
the order of -debug-only=si-wqm output and the naming of autonamed
values in -print-after=si-wqm output.

Reviewers: arsenm, rampitec, nhaehnle

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D74317