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

[Luke Lau via llvm-commits]

lukel97 wrote:

I'm not sure if there's much point in adding a cl::opt. Since nothing uses the metadata after #151483, and there's no flag to run EVLIndVarSimplifyPass anyway. So I don't think anyone is going to end up using it anyway.

Should we just go ahead and remove the metadata entirely?

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