First pass at emitIAS for branches and binding labels

src/assembler.h

Index: src/assembler.h
diff --git a/src/assembler.h b/src/assembler.h
index d769b3aef72a17c115d8fa976266934ddcd69b3b..81beb43f6f1c519cdb190670293b2e3c983eeb77 100644
--- a/src/assembler.h
+++ b/src/assembler.h
@@ -207,7 +207,7 @@ class Assembler {
 
 public:
   Assembler() {}
-  ~Assembler() {}
+  virtual ~Assembler() {}
 
   // Allocate a chunk of bytes using the per-Assembler allocator.
   uintptr_t AllocateBytes(size_t bytes) {
@@ -221,6 +221,8 @@ public:
     return reinterpret_cast<uintptr_t>(Allocator.Allocate(bytes, Alignment));
   }
 
+  virtual void BindCfgNodeLabel(SizeT NodeNumber) = 0;
+
   // Allocate data of type T using the per-Assembler allocator.
   template <typename T> T *Allocate() { return Allocator.Allocate<T>(); }