[SLP] NFC. Use InstructionsState::getOpcode only when necessary.#120210

HanKuanChen wants to merge 4 commits intollvm:mainllvm/llvm-project:mainfrom

HanKuanChen:slp-nfc-InstructionsState-use-getMainOpHanKuanChen/llvm-project:slp-nfc-InstructionsState-use-getMainOpCopy head branch name to clipboard