[llvm] [VPlan] Fix missing onlyFirstLaneUsed(). NFC (PR #145449)

[David Sherwood via llvm-commits]

david-arm wrote:

> Sorry for late response.
> 
> This patch is part of the #133213. Without inserting the `Broadcast` to the operand of recipes that should only accept scalar value cannot make test failure.

OK perhaps this is my fault then for asking you to split this patch out from #133213. I just assumed that the new code you added was possible to test using existing code paths?

https://github.com/llvm/llvm-project/pull/145449