Rewrite the compiler interface for CompilerDriver.

Change-Id: I15fa9afe7ffb7283ebda8d788a1e02793e3f75a6
diff --git a/compiler/dex/compiler_ir.h b/compiler/dex/compiler_ir.h
index 32fd79b..ea8eb1c 100644
--- a/compiler/dex/compiler_ir.h
+++ b/compiler/dex/compiler_ir.h
@@ -68,7 +68,7 @@
   uint32_t disable_opt;                // opt_control_vector flags.
   uint32_t enable_debug;               // debugControlVector flags.
   bool verbose;
-  CompilerBackend compiler_backend;
+  const CompilerBackend* compiler_backend;
   InstructionSet instruction_set;
 
   const InstructionSetFeatures& GetInstructionSetFeatures() {