[llvm] [WebAssembly] Combine shuffle and signed extend to extend_high (PR #179166)

[Sam Parker via llvm-commits]

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@@ -3699,6 +3699,34 @@ SDValue performConvertFPCombine(SDNode *N, SelectionDAG &DAG) {
   return SDValue();
 }
 
+static SDValue performExtendCombine(SDNode *N, SelectionDAG &DAG) {
+  SDValue In = N->getOperand(0);
+  // Look through bitcasts to find the underlying shuffle.
+  if (In.getOpcode() == ISD::BITCAST)
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sparker-arm wrote:

Maybe we just need to add support [here](https://github.com/llvm/llvm-project/blob/80662c1de160c94e93086f8337255e219752468c/llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp#L2290) for looking through bitcasts?

https://github.com/llvm/llvm-project/pull/179166