[AMDGPU] Fix test failures when expensive checks are enabled (#130644)

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.

Author: shiltian