[llvm] [VPlan][LV] Fix invalid truncation in VPScalarIVStepsRecipe (PR #137832)

[Florian Hahn via llvm-commits]

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

LGTM, thanks.

Please add  something like `Fixes https://github.com/llvm/llvm-project/issues/137185` to the PR description to make sure the issue is closed when this lands.

https://github.com/llvm/llvm-project/pull/137832