[llvm] [VPlan] Use SINGLE-SCALAR when printing single-scalar VPInstructions. (PR #141428)

[Luke Lau via llvm-commits]

https://github.com/lukel97 commented:

Should the VPInstructionWithType print changes also be included in this? https://github.com/llvm/llvm-project/pull/140623/commits/9c1a5dfda631447a7ff1cd293d8f68cd28765fd5#diff-34abe4c3cd34aa7a9664bbd204834248455635ba80b8a9ba9506d8c3e6b94d95R1084-R1090

https://github.com/llvm/llvm-project/pull/141428