This is a partial revert of D128830, restoring the previous position of DeadArgElim in the fat LTO pipeline. The motivation for this is a major code size regression observed in Rust and illustrated in the PhaseOrdering test.
This is a conservative fix restoring the previous pipeline order. The real problem is that the LTO pipeline is just broken: It doesn't have a CGSCC function simplification pipeline. The inliner is just being run by itself! This wouldn't be a problem if fat LTO used a standard design where ArgPromotion and DAE are only run after functions have already been simplified by the inliner pipeline.