Switch to incremental creation of the dispatcher

Before this patch the dispatcher area was created all at once at a final
stage, however it's useful also while translating, since it keeps all the
code reachable, which is particularly important to be able to build a
exhaustive dominator tree.

* Create the dispatcher area when a new instance of `JumpTargetManager`
  is created.
* Create a fake conditional branch to the dispatcher at the beginning of
  the `root` function.
* Incrementally build the dispatcher's switch case in
  `JumpTargetManager::getBlockAt`.

codegenerator.cpp

@@ -515,6 +515,16 @@ void CodeGenerator::translate(size_t LoadAddress,
   GlobalVariable *PCReg = Variables.getByEnvOffset(ptc.pc, "pc");
 
   JumpTargetManager JumpTargets(*TheModule, PCReg, MainFunction);
+
+  // Create a new block where the translation will start and emit a tautological
+  // branch to it, with false part being the dispatcher, which is always
+  // reachable
+  Entry = BasicBlock::Create(Context,
+                             "translation-start",
+                             MainFunction);
+  Builder.CreateCondBr(Builder.getTrue(), Entry, JumpTargets.dispatcher());
+
+
   std::map<std::string, BasicBlock *> LabeledBasicBlocks;
   std::vector<BasicBlock *> Blocks;