[llvm] [SelectionDAG] Simplify vselect true, T, F -> T (PR #100992)

[Simon Pilgrim via llvm-commits]

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@@ -13138,6 +13117,23 @@ SDNode *SelectionDAG::isConstantFPBuildVectorOrConstantFP(SDValue N) const {
   return nullptr;
 }
 
+std::optional<bool> SelectionDAG::isBoolConstant(SDValue N,
+                                                 bool AllowTruncation) const {
+  ConstantSDNode *Const = isConstOrConstSplat(N, false, AllowTruncation);
+  if (!Const)
+    return std::nullopt;
+
+  switch (TLI->getBooleanContents(N.getValueType())) {
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RKSimon wrote:

It might be safer if we only accept the 'clean' 1/0, -1/0 and 1b/0b T/F values and std::nullopt otherwise.

https://github.com/llvm/llvm-project/pull/100992