[llvm] [PowerPC] Fix handling of undefs in the PPC::isSplatShuffleMask query (PR #145149)

[via llvm-commits]

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@@ -2242,10 +2242,15 @@ bool PPC::isSplatShuffleMask(ShuffleVectorSDNode *N, unsigned EltSize) {
       return false;
 
   for (unsigned i = EltSize, e = 16; i != e; i += EltSize) {
-    if (N->getMaskElt(i) < 0) continue;
-    for (unsigned j = 0; j != EltSize; ++j)
-      if (N->getMaskElt(i+j) != N->getMaskElt(j))
-        return false;
+    // An UNDEF element is a sequence of UNDEF bits.
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RolandF77 wrote:

These are bytes, not bits.

https://github.com/llvm/llvm-project/pull/145149