[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?

OK, I've tried doing this, but I realised there are places where we have to invalidate the decision due to changes in the scalar epilogue status or interleave groups. However, that also means my first version was also incorrect even though all tests passed!

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