[llvm] [RISCV][TTI] Add shuffle costing for masked slide lowering (PR #128537)

[Alexey Bataev via llvm-commits]

================
@@ -413,6 +413,36 @@ bool llvm::getShuffleDemandedElts(int SrcWidth, ArrayRef<int> Mask,
   return true;
 }
 
+bool llvm::isMaskedSlidePair(ArrayRef<int> Mask, int NumElts,
+                             std::array<std::pair<int, int>, 2> &SrcInfo) {
+  const int SignalValue = NumElts * 2;
+  SrcInfo[0] = {-1, SignalValue};
+  SrcInfo[1] = {-1, SignalValue};
+  for (auto [i, M] : enumerate(Mask)) {
+    if (M < 0)
+      continue;
+    int Src = M >= (int)NumElts;
+    int Diff = (int)i - (M % NumElts);
+    bool Match = false;
+    for (int j = 0; j < 2; j++) {
+      auto &[SrcE, DiffE] = SrcInfo[j];
+      if (SrcE == -1) {
+        assert(DiffE == SignalValue);
+        SrcE = Src;
+        DiffE = Diff;
+      }
----------------
alexey-bataev wrote:

```suggestion
        DiffE = Diff;
        continue;
      }
```

https://github.com/llvm/llvm-project/pull/128537