[llvm] [DAG] Add utilities for inserting and extracting low sub-vectors [nfc] (PR #137230)

[Craig Topper via llvm-commits]

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@@ -924,6 +924,18 @@ class SelectionDAG {
   /// Example: shuffle A, B, <0,5,2,7> -> shuffle B, A, <4,1,6,3>
   SDValue getCommutedVectorShuffle(const ShuffleVectorSDNode &SV);
 
+  /// Insert SubVec into the lowest sub-vector of Vec.
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topperc wrote:

We could make it `getInsertSubvector/getExtractSubvector` with an `unsigned` index. Then we don't need `Low` and just pass 0 explicitly. I imagine that would let us replace even more users?

https://github.com/llvm/llvm-project/pull/137230