[llvm] [LV, VP]VP intrinsics support for the Loop Vectorizer + adding new tail-folding mode using EVL. (PR #76172)

[Alex Bradbury via llvm-commits]

asb wrote:

> Are there any plans on adding upstream runtime testing for EVL vectorization to guard against regressions?
> 
> We really should have upstream end-to-end testing that enables the EVL vectorization path and does a stage2 build + llvm-test-suite to catch regressions (similar to how SVE enabled bots were added when scalable vector support was added IIRC)

I'm currently working through a project to spin up a range of RISC-V builders. Part of that will involve deciding what configurations to test given the resources we have. It sounds like this could be an interesting config to add to the list. To clarify, is the suggestion basically a build with `-mllvm -force-tail-folding-style=data-with-evl`?

https://github.com/llvm/llvm-project/pull/76172