[llvm] [LV][EVL] Support fixed-order recurrence idiom with EVL tail folding. (PR #124093)

[Luke Lau via llvm-commits]

https://github.com/lukel97 approved this pull request.

LGTM

Not related to this PR but I'll keep an eye out for this on the codegen side: On RISC-V for a splice of -1, we should be able to do this as a single vslidedown I think?

https://github.com/llvm/llvm-project/pull/124093