Give sentinel traits the right to determine the policy where the sentinel is kept.

This should result in less indirect memory accesses, less dead writes and tighter code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@66061 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/BasicBlock.h

@@ -41,6 +41,10 @@ template<> struct ilist_traits<Instruction>
     return static_cast<Instruction*>(&Sentinel);
   }
   static void destroySentinel(Instruction*) {}
+
+  Instruction *provideInitialHead() const { return createSentinel(); }
+  Instruction *ensureHead(Instruction*) const { return createSentinel(); }
+
   static iplist<Instruction> &getList(BasicBlock *BB);
   static ValueSymbolTable *getSymTab(BasicBlock *ItemParent);
   static int getListOffset();