[llvm] [LV][EVL] Gate EVL loop metadata emission behind a cl::opt (PR #155760)

[Shih-Po Hung via llvm-commits]

arcbbb wrote:

> Could you add a rational for needing an option for this and why the default should be false?
> 
> My understanding is that the metadata is used somewhere and there should be no harm from adding it for all EVL vectorized loops.

Thanks. I’ve updated the PR description to include the rationale.

https://github.com/llvm/llvm-project/pull/155760