[llvm] [DAG] SelectionDAG::isKnownToBeAPowerOfTwo - add ISD::TRUNCATE handling and tests (PR #184365)

[Simon Pilgrim via llvm-commits]

================
@@ -4780,6 +4780,9 @@ bool SelectionDAG::isKnownToBeAPowerOfTwo(SDValue Val,
     // vscale(power-of-two) is a power-of-two
     return isKnownToBeAPowerOfTwo(Val.getOperand(0), /*OrZero=*/false,
                                   Depth + 1);
+
+  case ISD::TRUNCATE:
+    return isKnownToBeAPowerOfTwo(Val.getOperand(0), OrZero, Depth + 1);
----------------
RKSimon wrote:

@pranshoe alive2 proof?

https://github.com/llvm/llvm-project/pull/184365