[llvm] [VPlan] Convert EVL loops to variable-length stepping after dissolution (PR #147222)

[Shih-Po Hung via llvm-commits]

arcbbb wrote:

> Is the plan for this to replace https://github.com/llvm/llvm-project/pull/131005?

This is preparation work to enable uncountable EVL loops for speculative loads (#128593).

This approach should be able to replace #131005 since both accomplish the same goal, but it performs earlier in the vectorizer pipeline rather than at the end. 
I would say the pass runs post-VPlan optimization, so it won't affect vplan transformation.

https://github.com/llvm/llvm-project/pull/147222