[llvm] [VPlan] Move FOR splice cost into VPInstruction::FirstOrderRecurrenceSplice (PR #129645)

[Florian Hahn via llvm-commits]

fhahn wrote:

Looks like the issue was that the legacy cost model would treat first-order recurrence phis as forced scalars, but they cannot be scalarized, they will always generate a wide recipe. Pushed 41c1a7be3f1a2556e407e761acb766a5d103d691 to fix the legacy cost model to not force such phis as scalars.

https://github.com/llvm/llvm-project/pull/129645