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

[Michael Maitland via llvm-commits]

michaelmaitland wrote:

I have rebased and responded to the last round of reviews.

In the last round of reviews, I found that the data-with-evl cases were not getting handled as expected. Part of this was because I was missing `-prefer-predicate-over-epilogue=predicate-dont-vectorize` which overrides `data-with-evl`. I have added that option.

Once that option was added, I realized that we were unable to vectorize. Thats because the generic target does not support scalable vectorization nor predicated vectroization. We can enable scalable vectorization with `-scalable-vectorization=on`, but there is no way to make a generic target support predicated vectorization to my knowledge. As a result, I have moved these tests back into RISC-V. I think we can consider moving them back out until a generic target can handle it. 

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