[llvm] [VPlan] Fix miscompile after PR #142433. (PR #147398)

[James Y Knight via llvm-commits]

jyknight wrote:

Given that the comments are only on the test, and this is fixing a regression, I'm going to go ahead and merge after having resolved the comments as far as I can tell. If you have any additional comments, I'll happily fix with a follow-up.

https://github.com/llvm/llvm-project/pull/147398