[Pipelines] Restore old DAE position in LTO pipeline

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 conceptually 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
CGSCC inliner pipeline.

Differential Revision: https://reviews.llvm.org/D146051

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