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

[via llvm-commits]

appujee 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)
> 
> cc'ing some additional people who might also be able to help @appujee @Mel-Chen @nikolaypanchenko @arcbbb @preames

+1. Let me know if i can help with anything here.

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