[llvm] [SLP] NFC. Use InstructionsState::getOpcode only when necessary. (PR #120210)

[Alexey Bataev via llvm-commits]

alexey-bataev wrote:

> Use `isa_and_present` may be better than valid check + `getOpcode`. e.g., old code: `if (S.getOpcode() != Instruction::PHI || S.isAltShuffle())`
> 
> isa_and_present: `if (!isa_and_present<PHINode>(S.getMainOp()) || S.isAltShuffle())`
> 
> valid check: `if ((!S.valid() || S.getOpcode() != Instruction::PHI) || S.isAltShuffle())`

Better to have last (`valid` based) approach

https://github.com/llvm/llvm-project/pull/120210