[llvm] [SLP] NFC. Use InstructionsState::valid if users just want to know whether VL has same opcode. (PR #120217)

[Alexey Bataev via llvm-commits]

https://github.com/alexey-bataev commented:

getMainOp and getAltOp should assert if the state is invalid

https://github.com/llvm/llvm-project/pull/120217