Implement attributes for branch weight

[kvanbere]

The cold attribute exists for functions, but one might want to put weights on branches. This could have 3 forms; weight=N where N is 0-1, likely (1) and unlikely (0).

LLVM understands these annotations on branches already and can use these to fine-tune assembly to reduce branch mis-predictions. By nature, these could be used extensively in the standard library.

ie

match leaf {
    #[likely]
    Some(x) => recurse(x),
    None => {
        // do stuff that doesn't make sense to be in a cold fn
        ...
    }
}

There have been cases in Haskell libraries where changing the order of ADTs can increase performance by around 20% because branches are guessed differently by the compiler.

[pmarcelll]

#36181 added likely and unlikely intrinsics.

[cyplo]

Hi ! With #36181 being merged, it there any other changes needed to close this one ? Thank you !

[aturon]

Closing for the time being due to #36181.

[Eh2406]

There is a reference to this issue in the code:

rust/src/libstd/collections/hash/map.rs

Lines 141 to 143 in a86f556

	// Annotate exceedingly likely branches in `table::make_hash`
	// and `search_hashed` to reduce instruction cache pressure
	// and mispredictions once it becomes possible (blocked on issue #11092).

What needs to be done to remove that comment, now that this is closed?