[llvm] [VPlan] Fix miscompile when widening induction with loop-variant step (PR #206010)

[David Sherwood via llvm-commits]

================

----------------
david-arm wrote:

I checkout this PR and the code doesn't seem to have any effect on this test, i.e. I see the same IR with and without the changes in llvm/lib/Transforms/Vectorize/VPlanConstruction.cpp. Would you be able to restructure this PR so that the first commit adds the tests, and the second commit adds the code change so we can see the effect on the tests?

https://github.com/llvm/llvm-project/pull/206010