[llvm] [VPlan] Remove VPWidenRecipe constructor with no underlying instruction. NFCI (PR #166521)

[Ramkumar Ramachandra via llvm-commits]

https://github.com/artagnon approved this pull request.

Hm, can we also attempt to de-duplicate the opcodes between VPWidenRecipe and VPInstruction, as a follow-up? LGTM, thanks!

https://github.com/llvm/llvm-project/pull/166521