[llvm] [VPlan] Constant-fold ActiveLaneMask intrinsics (PR #208852)

[Yingwei Zheng via llvm-commits]

https://github.com/dtcxzyw requested changes to this pull request.

A correctness issue was identified by [llvm-hackme](https://github.com/dtcxzyw/llvm-hackme). Please see the [issue comment](https://github.com/llvm/llvm-project/pull/208852#issuecomment-5157944701) for details.

https://github.com/llvm/llvm-project/pull/208852