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

[Alexey Bataev via llvm-commits]

================
@@ -836,6 +836,8 @@ class InstructionsState {
     return getOpcode() == CheckedOpcode || getAltOpcode() == CheckedOpcode;
   }
 
+  bool valid() const { return MainOp != nullptr; }
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alexey-bataev wrote:

```suggestion
  /// Checks if the current state is valid, i.e. has non-null MainOp
  bool valid() const { return MainOp; }
```


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