[NFC] Add reverse iterator support for BasicBlocks in llvm::Function

[getianao]

Summary

In commit D140162, public access to BasicBlocks in llvm::Function was removed, including access to getBasicBlockList(). This revision aims to add reverse iterator support for BasicBlocks in llvm::Function to make it easier for developers to work with LLVM functions.

[aeubanks]

what about reverse(F)?

[barannikov88]

In D150449#4338653, @aeubanks wrote:

what about reverse(F)?

These methods might not be very useful, but having them for completeness won't hurt, I guess?

llvm/include/llvm/IR/Function.h

775

Please try to keep existing formatting.

[MaskRay]

+1 to reverse(F).

[getianao]

Fix format

llvm/include/llvm/IR/Function.h

 class LLVM_EXTERNAL_VISIBILITY Function : public GlobalObject,
                                           public ilist_node<Function> {
 public:
   using BasicBlockListType = SymbolTableList<BasicBlock>;
 
   // BasicBlock iterators...
   using iterator = BasicBlockListType::iterator;
   using const_iterator = BasicBlockListType::const_iterator;
+  using reverse_iterator = BasicBlockListType::reverse_iterator;
+  using const_reverse_iterator = BasicBlockListType::const_reverse_iterator;
 
   using arg_iterator = Argument *;
   using const_arg_iterator = const Argument *;
 
 private:
   // Important things that make up a function!
   BasicBlockListType BasicBlocks;         ///< The basic blocks
   mutable Argument *Arguments = nullptr;  ///< The formal arguments
public:
 
   //===--------------------------------------------------------------------===//
   // BasicBlock iterator forwarding functions
   //
   iterator                begin()       { return BasicBlocks.begin(); }
   const_iterator          begin() const { return BasicBlocks.begin(); }
   iterator                end  ()       { return BasicBlocks.end();   }
   const_iterator          end  () const { return BasicBlocks.end();   }
+  reverse_iterator       rbegin()       { return BasicBlocks.rbegin();}
+  const_reverse_iterator rbegin() const { return BasicBlocks.rbegin();}
+  reverse_iterator         rend()       { return BasicBlocks.rend();  }
+  const_reverse_iterator   rend() const { return BasicBlocks.rend();  }
 
   size_t                   size() const { return BasicBlocks.size();  }
   bool                    empty() const { return BasicBlocks.empty(); }
   const BasicBlock       &front() const { return BasicBlocks.front(); }
         BasicBlock       &front()       { return BasicBlocks.front(); }
   const BasicBlock        &back() const { return BasicBlocks.back();  }
         BasicBlock        &back()       { return BasicBlocks.back();  }
 
 /// @name Function Argument Iteration