Add command line support for enabling the optimizing compiler.

Also run tests with the optimizing compiler enabled when
the file art/USE_OPTIMIZING_COMPILER is present.

Change-Id: Ibc33eed62a43547bc3b9fe786d014c0d81b5add8
diff --git a/compiler/dex/compiler_ir.h b/compiler/dex/compiler_ir.h
index c71f047..70159ca 100644
--- a/compiler/dex/compiler_ir.h
+++ b/compiler/dex/compiler_ir.h
@@ -62,7 +62,7 @@
   uint32_t disable_opt;                // opt_control_vector flags.
   uint32_t enable_debug;               // debugControlVector flags.
   bool verbose;
-  const CompilerBackend* compiler_backend;
+  const Compiler* compiler;
   InstructionSet instruction_set;
   bool target64;