[llvm] [VPlan] Model FOR extract of exit value in VPlan. (PR #93395)

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

Addressed latest comments, thanks!

Added new `isVectorToScalar` as suggested.

> Operator- would naturally return an ElementCount
An issue with that may be that we would need to create a VPLane from the element count first, to apply it's operator-, which may be almost as verbose as currently, which is a bit more explicit.

https://github.com/llvm/llvm-project/pull/93395