[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 when we call `newTreeEntry`, we don't provide the `MainOp` and `AltOp`.
```
  TreeEntry *newTreeEntry(ArrayRef<Value *> VL,
                          TreeEntry::EntryState EntryState,
                          std::optional<ScheduleData *> Bundle,
                          const InstructionsState &S,
                          const EdgeInfo &UserTreeIdx,
                          ArrayRef<int> ReuseShuffleIndices = {},
                          ArrayRef<unsigned> ReorderIndices = {}) {
    assert(((!Bundle && EntryState == TreeEntry::NeedToGather) ||
            (Bundle && EntryState != TreeEntry::NeedToGather)) &&
           "Need to vectorize gather entry?");
    // Gathered loads still gathered? Do not create entry, use the original one.
    if (GatheredLoadsEntriesFirst.has_value() &&
        EntryState == TreeEntry::NeedToGather &&
        S.getOpcode() == Instruction::Load && UserTreeIdx.EdgeIdx == UINT_MAX &&
        !UserTreeIdx.UserTE)
      return nullptr;
    VectorizableTree.push_back(std::make_unique<TreeEntry>(VectorizableTree));
    TreeEntry *Last = VectorizableTree.back().get();
```
It will make `TreeEntry` call the default constructor of `InstructionsState`. However, it is deleted.

This patch will make `InstructionsState` can be constructed by default constructor.

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