[llvm] [RISCV] Allow undef prefix for local repeating VLA shuffle lowering (PR #126097)

[Craig Topper via llvm-commits]

================
@@ -5338,13 +5338,19 @@ static SDValue lowerDisjointIndicesShuffle(ShuffleVectorSDNode *SVN,
 /// Is this mask local (i.e. elements only move within their local span), and
 /// repeating (that is, the same rearrangement is being done within each span)?
 static bool isLocalRepeatingShuffle(ArrayRef<int> Mask, int Span) {
-  // TODO: Could improve the case where undef elements exist in the first span.
+  SmallVector<int> LowSpan(Span, -1);
   for (auto [I, M] : enumerate(Mask)) {
     if (M == -1)
       continue;
-    int ChunkLo = I - (I % Span);
+    int SpanIdx = I % Span;
+    int ChunkLo = I - SpanIdx;
     int ChunkHi = ChunkLo + Span;
-    if (M < ChunkLo || M >= ChunkHi || M - ChunkLo != Mask[I % Span])
+    if (M < ChunkLo || M >= ChunkHi)
----------------
topperc wrote:

This can be simplified to

```
if ((M / Span) != (I / Span))
  return false;
int Expected = M % Span;
```

https://github.com/llvm/llvm-project/pull/126097