[llvm] [ValueTracking][SelectionDAG] Use KnownBits::reverseBits/byteSwap. NFC (PR #155847)

[Simon Pilgrim via llvm-commits]

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@@ -1834,13 +1834,11 @@ static void computeKnownBitsFromOperator(const Operator *I,
       }
       case Intrinsic::bitreverse:
         computeKnownBits(I->getOperand(0), DemandedElts, Known2, Q, Depth + 1);
-        Known.Zero |= Known2.Zero.reverseBits();
-        Known.One |= Known2.One.reverseBits();
+        Known = Known.unionWith(Known2.reverseBits());
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RKSimon wrote:

or why isn't the abs case `Known = Known.unionWith(Known2.abs(IntMinIsPoison));`?

https://github.com/llvm/llvm-project/pull/155847