[llvm] [LV] Ensure VPInstruction::usesFirstLaneOnly behaves correctly for WidePtrAdd (PR #169344)

[Luke Lau via llvm-commits]

lukel97 wrote:

> Hmm, for a scalar plan, we should probably not emit a wide pointer add in the first place I think?

Yeah this crossed my mind too, namely since a wideptradd should only be emitted by a VPWidenPointerInduction in convertToConcrete recipes. But I wasn't sure if legalizeAndOptimizeInductions is guaranteed to remove them all.

Btw this is a recent crash, I can't recreate this on 86a82f27ee8cb7477c25561bc5697a6a6a8ff21d

https://github.com/llvm/llvm-project/pull/169344