[llvm] [VPlan] Split VPWidenMemoryInstructionRecipe (NFCI). (PR #87411)

[via llvm-commits]

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

LGTM, thanks! Please wait a day or so if @alexey-bataev has further comments.
Added minor nits.
Commit message worth updating: last paragraph regarding operand reordering, and slight typo in first paragraph.

https://github.com/llvm/llvm-project/pull/87411