[llvm] [GISEL] Add IRTranslation for shufflevector on scalable vector types (PR #80378)

[David Green via llvm-commits]

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@@ -639,6 +639,11 @@ Concatenate two vectors and shuffle the elements according to the mask operand.
 The mask operand should be an IR Constant which exactly matches the
 corresponding mask for the IR shufflevector instruction.
 
+G_SPLAT_VECTOR
+^^^^^^^^^^^^^^^^
+
+Create a vector where all elements are the scalar from the source operand.
+
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davemgreen wrote:

There is a ISD::SPLAT_VECTOR in DAG, used for scalable vectors. I was contemplating using G_BUILD_VECTOR for all splats in AArch64, to remove the current G_DUP and G_DUPLANE's.

https://github.com/llvm/llvm-project/pull/80378