[llvm] [RISCV] Initial codegen support for the XRivosVizip extension (PR #131933)

[Luke Lau via llvm-commits]

================
@@ -4611,6 +4613,43 @@ static bool isElementRotate(std::array<std::pair<int, int>, 2> &SrcInfo,
          SrcInfo[1].second - SrcInfo[0].second == (int)NumElts;
 }
 
+static bool isAlternating(std::array<std::pair<int, int>, 2> &SrcInfo,
+                          ArrayRef<int> Mask, bool &Polarity) {
+  int NumElts = Mask.size();
+  bool NonUndefFound = false;
+  for (unsigned i = 0; i != Mask.size(); ++i) {
+    int M = Mask[i];
+    if (M < 0)
+      continue;
+    int Src = M >= (int)NumElts;
+    int Diff = (int)i - (M % NumElts);
+    bool C = Src == SrcInfo[1].first && Diff == SrcInfo[1].second;
----------------
lukel97 wrote:

Just to check my understanding, can we assert that

```c++
assert((Src == SrcInfo[0].first && Diff = SrcInfo[0].second) || (Src == SrcInfo[1].first && Diff == SrcInfo[1].second));
```

https://github.com/llvm/llvm-project/pull/131933