Better choice of LLVM optimization passes

[DemiMarie]

LLVM's -O2 and -O3 pipelines are tuned for C and C++ and Rust can almost certainly do better with a custom ordering.

[rkruppe]

See https://internals.rust-lang.org/t/using-a-custom-optimisation-pass-pipeline/2345 for prior discussion

[Mark-Simulacrum]

I feel like there's nothing concrete to be done here, and discussions such as these are better left for internals.rust-lang.org, so I'm going to close.