Use clear() to zero an existing APInt.

llvm-svn: 52601
diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index bca5e75..abd7272 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -1406,7 +1406,7 @@
   case ISD::CTPOP: {
     unsigned LowBits = Log2_32(BitWidth)+1;
     KnownZero = APInt::getHighBitsSet(BitWidth, BitWidth - LowBits);
-    KnownOne  = APInt(BitWidth, 0);
+    KnownOne.clear();
     return;
   }
   case ISD::LOAD: {