[llvm] [LV][VPlan] Add initial support for CSA vectorization (PR #106560)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

> This PR contains a series of commits which together implement an initial version of conditional scalar vectorization.
> 
> A detailed explanation of the changes can be found [here](https://discourse.llvm.org/t/vectorization-of-conditional-scalar-assignment-csa/80964).

Kindly note that the only functionality is squash-and-merge, and this will be the final commit message: "series of commits" doesn't make sense in the final commit message. Also, it would be nice to have a short explanation in the commit message itself (possibly with a few examples). Finally, kindly ensure that markdown doesn't end up when someone does a `git log`: it's plain-text.

https://github.com/llvm/llvm-project/pull/106560