Add an explicit bool operator to SDValue to make it easier to test for

a non-null node. In particular, this makes it easier to use condition
variables with SDValues, etc.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212323 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/SelectionDAGNodes.h

@@ -142,6 +142,9 @@ public:
   bool operator<(const SDValue &O) const {
     return std::tie(Node, ResNo) < std::tie(O.Node, O.ResNo);
   }
+  LLVM_EXPLICIT operator bool() const {
+    return Node != nullptr;
+  }
 
   SDValue getValue(unsigned R) const {
     return SDValue(Node, R);