[llvm] [LV][EVL] Introduce the EVLIndVarSimplify Pass for EVL-vectorized loops (PR #131005)

[Min-Yih Hsu via llvm-commits]

mshockwave wrote:

> > The former, also known as canonical IV, is more favorable for analyses as it's "countable" in the sense of SCEV
> 
> I presume in comparison to #91796 the canonical IV now has a much shorter lifetime. Do we know what analyses still use the canonical IV in between the loop vectorizer & the end of vectorization?
> 
> EDIT: I've answered my own question, from opt-pipeline-vector-passes.ll:
> 
> ```
> Running pass: LoopVectorizePass on f (11 instructions)
> Running analysis: DemandedBitsAnalysis on f
> Running pass: InferAlignmentPass on f (11 instructions)
> Running pass: LoopLoadEliminationPass on f (11 instructions)
> Running pass: InstCombinePass on f (11 instructions)
> Running analysis: LastRunTrackingAnalysis on f
> Running pass: SimplifyCFGPass on f (11 instructions)
> Running pass: SLPVectorizerPass on f (11 instructions)
> Running pass: VectorCombinePass on f (11 instructions)
> Running pass: InstCombinePass on f (11 instructions)
> Running pass: LoopUnrollPass on f (11 instructions)
> Running pass: WarnMissedTransformationsPass on f (11 instructions)
> Running pass: SROAPass on f (11 instructions)
> Running pass: InferAlignmentPass on f (11 instructions)
> Running pass: InstCombinePass on f (11 instructions)
> Running pass: LoopSimplifyPass on f (11 instructions)
> Running pass: LCSSAPass on f (11 instructions)
> Running analysis: MemorySSAAnalysis on f
> Running pass: LICMPass on loop %loop in function f
> Running pass: AlignmentFromAssumptionsPass on f (11 instructions)
> ```

To give another data point: I actually tried to put this Pass right after the LoopVectorized, and in that case it only brought ~3% improvement on the dynamic IC of 401.bzip2, instead of the 8% improvement we see now.

https://github.com/llvm/llvm-project/pull/131005