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

[Simon Pilgrim 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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RKSimon wrote:

Sorry for the bike shedding - but maybe just getWidenSubvector/getNarrowSubvector and just handle the (INSERT_SUBVECTOR undef, sub, 0) case for getWidenSubvector?

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