[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;
+    if (!NonUndefFound) {
+      NonUndefFound = true;
+      Polarity = (C == i % 2);
+      continue;
+    }
+    if ((Polarity && C != i % 2) || (!Polarity && C == i % 2))
+      return false;
+  }
+  return true;
+}
+
+static bool isZipEven(std::array<std::pair<int, int>, 2> &SrcInfo,
----------------
lukel97 wrote:

Nit, it took me a little while to remember the fields of SrcInfo again, for this + isZipOdd do you think it would be useful to add a comment showing like:

```
Src0: a0 a1 a2 a3
Src1:    b0 b1 b2 b3
-----------------
Res:  a0 b0 a2 b2
```

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