LLVM/MIR optimizations to simplify the new RFC3058 `?` desugaring

[scottmcm]

With #84767 implementing the new ? desugaring, there's no mir-opt that can simplify the double-match it emits, as the arm identity pass only does Result->Result, and thus cannot handle Result->ControlFlow->Result.

That's not a blocker for checkin, as the arm identity pass is disabled by default currently because of things like #78628 and it depends on the MIR inliner being enabled anyway, but it sure would be nice to do better here.

Zulip conversation with some more context: https://rust-lang.zulipchat.com/#narrow/stream/189540-t-compiler.2Fwg-mir-opt/topic/Suggestions.20for.20a.20beginner/near/237739360

[Moxinilian]

I am interested in taking this one as a first contribution. I will probably have time to work on it this week.

[Moxinilian]

For interested people, conversation on Zulip moved to https://rust-lang.zulipchat.com/#narrow/stream/189540-t-compiler.2Fwg-mir-opt/topic/Optimizing.20a.20match.20on.20a.20just-constructed.20enum.20.2385133

[michaelmaitland]

I see this issue marked as open. Is it being worked on or available for me to pickup?

[Moxinilian]

It is being worked on, see #85646. Any suggestion/help is welcome, though!