[llvm] [VectorUtils] Add llvm::scaleShuffleMaskElts wrapper for narrowShuffleMaskElts/widenShuffleMaskElts (PR #96646)

[Yingwei Zheng via llvm-commits]

================
@@ -418,6 +418,31 @@ bool llvm::widenShuffleMaskElts(int Scale, ArrayRef<int> Mask,
   return true;
 }
 
+bool llvm::scaleShuffleMaskElts(unsigned NumDstElts, ArrayRef<int> Mask,
+                                SmallVectorImpl<int> &ScaledMask) {
+  unsigned NumSrcElts = Mask.size();
+  assert(NumSrcElts > 0 && NumDstElts > 0 && "Unexpected scaling factor");
+
+  // Fast-path: if no scaling, then it is just a copy.
+  if (NumSrcElts == NumDstElts) {
+    ScaledMask.assign(Mask.begin(), Mask.end());
+    return true;
+  }
+
+  // We can't find a whole scale factor - bail out.
+  if ((NumSrcElts % NumDstElts) != 0 && (NumDstElts % NumSrcElts) != 0)
----------------
dtcxzyw wrote:

This should be an assertion.


https://github.com/llvm/llvm-project/pull/96646