intrusive_list: Add size querying

Since we store pointers and have an interface for iterators
set up, the count is just the distance from the beginning
to the end of the list.

Nice thing is that because of this, basic blocks also get
the ability to have a size count without needing to do anything
directly.

src/frontend/ir/basic_block.h

@@ -30,6 +30,7 @@ namespace IR {
 class Block final {
 public:
     using InstructionList        = Common::IntrusiveList<Inst>;
+    using size_type              = InstructionList::size_type;
     using iterator               = InstructionList::iterator;
     using const_iterator         = InstructionList::const_iterator;
     using reverse_iterator       = InstructionList::reverse_iterator;
@@ -38,6 +39,7 @@ public:
     explicit Block(const Arm::LocationDescriptor& location) : location(location) {}
 
     bool                   empty()   const { return instructions.empty();   }
+    size_type              size()    const { return instructions.size();    }
 
     Inst&                  front()         { return instructions.front();   }
     const Inst&            front()   const { return instructions.front();   }