[llvm] [GlobalISel] Make scalar G_SHUFFLE_VECTOR illegal. (PR #140508)

[Amara Emerson via llvm-commits]

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@@ -3330,6 +3330,51 @@ bool IRTranslator::translateShuffleVector(const User &U,
     Mask = SVI->getShuffleMask();
   else
     Mask = cast<ConstantExpr>(U).getShuffleMask();
+
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aemerson wrote:

For future readers some comments here would be useful to quickly parse that the code is trying to do.

https://github.com/llvm/llvm-project/pull/140508