document MIR-optimizations

[JohnTitor]

Original discussion is in Zulip.

At now, src/mir/optimizations.md is empty so we should document MIR-optimizations.

oli-obk pointed out these:

• guaranteed and required ones
• some that don't optimize, but just check stuff
• some that are only turned on in release mode

[mark-i-m]

@JohnTitor Would you be interested in submitting a PR to add these. Frankly, outside the WG-mir-opt, there are probably not many people qualified to write it.

[JohnTitor]

@mark-i-m Yes, I'll work on this. But it may take some time to come up with the suitable content.

[oli-obk]

The list of mir optimizations inside the compiler can be found at https://github.com/rust-lang/rust/blob/2a9be46cc4b5aa09e978653aba071c7c52b531eb/src/librustc_mir/transform/mod.rs#L232-L282

[spastorino]

I've opened #517 which is related :)