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

[Shih-Po Hung via llvm-commits]

arcbbb wrote:

@fhahn while there have been earlier attempts to make the EVL loop uncountable, this is the first one undertaken after dissolving the structured loop form. Does this align with the intended VPlan design?

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