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

[Michael Maitland via llvm-commits]

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@@ -423,6 +424,61 @@ class InductionDescriptor {
   SmallVector<Instruction *, 2> RedundantCasts;
 };
 
+/// A Conditional Scalar Assignment (CSA) is an assignment from an initial
+/// scalar that may or may not occur.
+class CSADescriptor {
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michaelmaitland wrote:

Intel has used the term [conditional scalar assingmnet](https://www.intel.com/content/www/us/en/developer/articles/troubleshooting/fdiag15336.html). I have abbreviated it as CSA for short. I have documented the acronym in the code in this patch in multiple places

> /// A Conditional Scalar Assignment (CSA) is an assignment from an initial
> /// scalar that may or may not occur.

> // This file "describes" induction, recurrence, and conditional scalar
> // assignment (CSA) variables.

> STATISTIC(CSAsVectorized,
>          "Number of conditional scalar assignments vectorized");

I thought that `ConditionalScalarAssignmentDescriptor`, `createConditionalScalarAssignmentMaskPhi`, and `VPConditionalScalarAssignmentDescriptorExtractScalarRecipe` were quite long for example. 

Do you have any suggestion on what you'd like it to be named? Is expanding `CSA` to `ConditionalScalarAssignment` everywhere your preference?

For now, I've tried to be proactive in ab17128ac4e8176a7b4dde5677759c1ba8371836.

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