Investigate performance benefits from mir-opt-level

[joshlf]

Rust currently supports an unstable option, mir-opt-level (analogous to opt-level) that instructs Rust to perform optimizations at the MIR level. We should investigate whether this gives us any performance improvements over simply using the stable opt-level.

[joshlf]

We've run into some ICEs:

• ICE: unexpected panic with mir-opt-level=3 rust-lang/rust#45493
• ICE: Unexpected panic with mir-opt-level=3 rust-lang/rust#45975