Enable mir inlining by default

[bjorn3]

Blocked on:

• ICE trying to compile libstd with -Zmir-opt-level=3 rust#63802
• passing reference to field of by-value struct causes ICE with mir-opt-level=3 rust#66345
• Miscompilation with -Zmir-opt-level=2 rust#67529
• maybe ICE on ./mir-opt/inline/inline-into-box-place.rs with -Zmir-opt-level=3 : Return place is None, not local rust#67550
• maybe MIR inliner ICEs trying to inline async closures rust#68841
• Tracking Issue for mir-inlining rust#81567 is the upstream tracking issue for enabling the mir inliner

Cranelift doesn't have any inlining support itself.

Edit(2019-12-23): Added more blocking issues
Edit(2021-01-31): Added upstream tracking issue

[bjorn3]

bjorn3@84f24e5 re-enables mir inlining again for libstd. This resulted in a perf improvement of about 30% on simple-raytracer.

[bjorn3]

Since rust-lang/rust#91743 the MIR inliner is enabled by default in release mode. Given that the optimized MIR is stored in the crate metadata, having the standard library compiled with MIR inlining enabled benefits even debug mode user programs.