[llvm] [WASM] Fold bitselect with splat zero (PR #147305)

[via llvm-commits]

================
@@ -3210,6 +3213,64 @@ static SDValue performTruncateCombine(SDNode *N,
   return truncateVectorWithNARROW(OutVT, In, DL, DAG);
 }
 
+static SDValue performVSelectCombine(SDNode *N, SelectionDAG &DAG) {
+  // In the tablegen.td, vselect A B C -> bitselect B C A
+
+  // SCENARIO A
+  // vselect <0>, X, Y
+  // -> bitselect X, Y, <0>
+  // -> or (AND(X, <0>), AND(<Y>, !<0>))
+  // -> or (0, AND(<Y>, !<0>))
+  // -> AND(Y, !<0>)
+  // -> AND(Y, 1)
+  // -> Y
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badumbatish wrote:

we do have it as, I will remove the redundant checks

```
SDValue SelectionDAG::simplifySelect(SDValue Cond, SDValue T, SDValue F) {
  // select undef, T, F --> T (if T is a constant), otherwise F
  // select, ?, undef, F --> F
  // select, ?, T, undef --> T
  if (Cond.isUndef())
    return isConstantValueOfAnyType(T) ? T : F;
  if (T.isUndef())
    return F;
  if (F.isUndef())
    return T;

  // select true, T, F --> T
  // select false, T, F --> F
  if (auto C = isBoolConstant(Cond, /*AllowTruncation=*/true))
    return *C ? T : F;

  // select ?, T, T --> T
  if (T == F)
    return T;

  return SDValue();
}
```

https://github.com/llvm/llvm-project/pull/147305