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

[Florian Hahn via llvm-commits]

fhahn wrote:

Looks like this has been crashing for a couple of hours.

Can we revert this to unblock people or land a fix ASAP?

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