[llvm] [VPlan] Don't apply predication discount to non-originally-predicated blocks (PR #160449)

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

Do you by any chance have any perf numbers to confirm this doesn't cause any regressions?

If so, would be great if you could add them to the PR description

https://github.com/llvm/llvm-project/pull/160449