[llvm] [SLP] NFC: Remove redundant expression from getBuildVectorCost (PR #222547)

[Alexey Bataev via llvm-commits]

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alexey-bataev wrote:

Not NFC, lleaves the mask as 0, before it was the position. We prefer to have the position insytead of 0, reduces number of per-lane movements

https://github.com/llvm/llvm-project/pull/222547