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

[Philip Reames 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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preames wrote:

I would find these names extremely confusing, as widen and narrow would be things I'd expect to be done element wise to the elements given those names.   I'm fine exploring the case with a undef passthru (that is fairly common), but these names really don't work.  

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