This PR fixes test failures introduced in #127353 when expensive checks are enabled. For `llvm/test/CodeGen/AMDGPU/materialize-frame-index-sgpr.ll` and `llvm/test/CodeGen/AMDGPU/materialize-frame-index-sgpr.gfx10.ll`, `s59` is no longer in live-ins because it is caller saved. Switch to `s55` in this PR.