[llvm] [NFC][LoopVectorize] Cache result of requiresScalarEpilogue (PR #108981)

[David Sherwood via llvm-commits]

david-arm wrote:

> Would it be simpler to have a separate helper `setRequiresScalarEpilogue` that's called once up-front with `requiresScalarEpilogue` simply returning the decision as done in multiple other places?

I'm happy to do it this way, but I couldn't convince myself of the earliest place to put such code that is guaranteed not to crash. I can go off and have a look of course, but if you happen to have any ideas that would be great!

https://github.com/llvm/llvm-project/pull/108981