[llvm] [SLP] NFC. Replace MainOp and AltOp in TreeEntry with InstructionsState. (PR #120198)

[Han-Kuan Chen via llvm-commits]

================
@@ -836,7 +836,7 @@ class InstructionsState {
     return getOpcode() == CheckedOpcode || getAltOpcode() == CheckedOpcode;
   }
 
-  InstructionsState() = delete;
+  InstructionsState() = default;
----------------
HanKuanChen wrote:

The problem is the constructor of `TreeEntry`.
```
  struct TreeEntry {
    using VecTreeTy = SmallVector<std::unique_ptr<TreeEntry>, 8>;
    TreeEntry(VecTreeTy &Container) : Container(Container) {}
```
`Container` is just used to dump the SLP tree. See 95da05c3f5064c733dc486a43f262a55deb03dc5.

https://github.com/llvm/llvm-project/pull/120198