Guard against self-assignment in the DominatorTreeBase move assignment operator. (#116464)

The `DominatorTreeBase` move assignment operator was not self-assignment
safe because the last thing it does before returning is to release all
resources held by the source object. This issue was reported by a static
analysis tool; no self-assignment is known to occur in practice.

Author: tahonermann

llvm/include/llvm/Support/GenericDomTree.h

@@ -287,6 +287,8 @@ class DominatorTreeBase {
   }
 
   DominatorTreeBase &operator=(DominatorTreeBase &&RHS) {
+    if (this == &RHS)
+      return *this;
     Roots = std::move(RHS.Roots);
     DomTreeNodes = std::move(RHS.DomTreeNodes);
     NodeNumberMap = std::move(RHS.NodeNumberMap);