[llvm] VectorCombine: refactor foldShuffleToIdentity (NFC) (PR #92766)

[Ramkumar Ramachandra via llvm-commits]

================
@@ -1668,6 +1668,86 @@ bool VectorCombine::foldShuffleOfShuffles(Instruction &I) {
   return true;
 }
 
+using InstLane = std::pair<Value *, int>;
+
+static InstLane lookThroughShuffles(Value *V, int Lane) {
+  while (auto *SV = dyn_cast<ShuffleVectorInst>(V)) {
+    unsigned NumElts =
+        cast<FixedVectorType>(SV->getOperand(0)->getType())->getNumElements();
+    int M = SV->getMaskValue(Lane);
+    if (M < 0)
+      return {nullptr, PoisonMaskElem};
+    if (static_cast<unsigned>(M) < NumElts) {
+      V = SV->getOperand(0);
+      Lane = M;
+    } else {
+      V = SV->getOperand(1);
+      Lane = M - NumElts;
+    }
+  }
+  return InstLane{V, Lane};
+}
+
+static SmallVector<InstLane>
+generateInstLaneVectorFromOperand(ArrayRef<InstLane> Item, int Op) {
+  SmallVector<InstLane> NItem;
+  for (InstLane IL : Item) {
+    auto [V, Lane] = IL;
+    InstLane OpLane =
+        V ? lookThroughShuffles(cast<Instruction>(V)->getOperand(Op), Lane)
+          : InstLane{nullptr, PoisonMaskElem};
+    NItem.emplace_back(OpLane);
+  }
+  return NItem;
+}
+
+static Value *generateNewInstTree(ArrayRef<InstLane> Item, FixedVectorType *Ty,
+                                  const SmallPtrSet<Value *, 4> &IdentityLeafs,
+                                  const SmallPtrSet<Value *, 4> &SplatLeafs,
+                                  IRBuilder<> &Builder) {
+  auto [FrontV, FrontLane] = Item.front();
+
+  if (IdentityLeafs.contains(FrontV) &&
+      all_of(drop_begin(enumerate(Item)), [Item](const auto &E) {
+        Value *FrontV = Item.front().first;
----------------
artagnon wrote:

>From https://en.cppreference.com/w/cpp/language/structured_binding:

```cpp
// Structured bindings cannot be captured by lambda expressions:

#include <cassert>
 
int main()
{
    struct S { int p{6}, q{7}; };
    const auto& [b, d] = S{};
    auto l = [b, d] { return b * d; }; // valid since C++20
    [assert](http://en.cppreference.com/w/cpp/error/assert)(l() == 42);
}
```

Very unintuitive and ugly special-case, I know.

https://github.com/llvm/llvm-project/pull/92766