[llvm] [DAG] Fold build_vector(build_pair()) patterns. (PR #88261)

[Matt Arsenault via llvm-commits]

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@@ -23497,6 +23497,29 @@ SDValue DAGCombiner::visitBUILD_VECTOR(SDNode *N) {
       return DAG.getNode(ISD::SPLAT_VECTOR, SDLoc(N), VT, V);
     }
 
+  // build_vector(build_pair(x,y)) -> bitcast(build_pair(x,y))
+  if (N->getNumOperands() == 1 &&
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arsenm wrote:

I forgot 1 element build_vector is even valid 

https://github.com/llvm/llvm-project/pull/88261