[llvm] [LLVM][DAGCombiner][SVE] Fold vselect into merge_pasthru_op. (PR #146917)

[Paul Walker via llvm-commits]

================
@@ -25531,6 +25534,22 @@ static SDValue performVSelectCombine(SDNode *N, SelectionDAG &DAG) {
   if (isAllInactivePredicate(N0))
     return N->getOperand(2);
 
+  if (isMergePassthruOpcode(IfTrue.getOpcode()) && IfTrue.hasOneUse()) {
+    // vselect A, (merge_pasthru_op all_active, B,{Bn,} -), C
+    // vselect A, (merge_pasthru_op -, B,{Bn,} undef), C
+    // vselect A, (merge_pasthru_op A, B,{Bn,} -), C
+    //   -> merge_pasthru_op A, B,{Bn,} C
+    if (isAllActivePredicate(DAG, IfTrue->getOperand(0)) ||
+        IfTrue->getOperand(IfTrue.getNumOperands() - 1).isUndef() ||
+        IfTrue->getOperand(0) == N0) {
----------------
paulwalker-arm wrote:

Whilst striving to extend the isel test coverage I've forgotten to test the DAG combine itself.  I'll add tests to trigger the three cases.

https://github.com/llvm/llvm-project/pull/146917