Specialize StepBy<Range(Inclusive)>
Part of #51557, related to #43064, #31155
As discussed in the above issues, `step_by` optimizes very badly on ranges which is related to
1. the special casing of the first `StepBy::next()` call
2. the need to do 2 additions of `n - 1` and `1` inside the range's `next()`
This PR eliminates both by overriding `next()` to always produce the current element and also step ahead by `n` elements in one go. The generated code is much better, even identical in the case of a `Range` with constant `start` and `end` where `start+step` can't overflow. Without constant bounds it's a bit longer than the manual loop. `RangeInclusive` doesn't optimize as nicely but is still much better than the original asm.
Unsigned integers optimize better than signed ones for some reason.
See the following two links for a comparison.
[godbolt: specialization for ..](https://godbolt.org/g/haHLJr)
[godbolt: specialization for ..=](https://godbolt.org/g/ewyMu6)
`RangeFrom`, the only other range with an `Iterator` implementation can't be specialized like this without changing behaviour due to overflow. There is no way to save "finished-ness".
The approach can not be used in general, because it would produce side effects of the underlying iterator too early.
May obsolete #51435, haven't checked.
95979dcfb2