[llvm] [VPlan] Narrow truncates of an induction plus a constant offset (PR #226035)

[Ramkumar Ramachandra via llvm-commits]

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artagnon wrote:

In this particular case, having a non-constant post-inc Start/Offset would not be _profitable_, and we should check that here, where it's clear?

https://github.com/llvm/llvm-project/pull/226035