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

[Alexey Bataev via llvm-commits]

================
@@ -821,7 +821,8 @@ class InstructionsState {
 
   /// The main/alternate opcodes for the list of instructions.
   unsigned getOpcode() const {
-    return MainOp ? MainOp->getOpcode() : 0;
+    assert(MainOp && "InstructionsState is invalid.");
----------------
alexey-bataev wrote:

```suggestion
    assert(valid() && "InstructionsState is invalid.");
```


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