[llvm] [LV][EVL] Support reduction idioms using tail folding with EVL. (PR #90184)

[Kolya Panchenko via llvm-commits]

nikolaypanchenko wrote:

> any more thoughts about runtime testing and why the current runtime testing didn't uncover the issues mentioned?

There are few problems to catch such things:
1. At least we don't have HW that supports VL selection not as `vlmax + rest`
2. Upstream QEMU has no option to select other than `vlmax + rest`
3. Our downstream QEMU does have that option and we caught few issues like that, but we don't test upstream LLVM.

@asb is building buildbots and when QEMU option will be available in upstream, that option will be added there too.

https://github.com/llvm/llvm-project/pull/90184