ART: Some Quick cleanup

Make several fields const in CompilationUnit. May benefit some Mir2Lir
code that repeats tests, and in general immutability is good.

Remove compiler_internals.h and refactor some other headers to reduce
overly broad imports (and thus forced recompiles on changes).

Change-Id: I898405907c68923581373b5981d8a85d2e5d185a
diff --git a/compiler/dex/bb_optimizations.h b/compiler/dex/bb_optimizations.h
index aac2644..7685200 100644
--- a/compiler/dex/bb_optimizations.h
+++ b/compiler/dex/bb_optimizations.h
@@ -18,8 +18,10 @@
 #define ART_COMPILER_DEX_BB_OPTIMIZATIONS_H_
 
 #include "base/casts.h"
-#include "compiler_internals.h"
+#include "compiler_ir.h"
+#include "dex_flags.h"
 #include "pass_me.h"
+#include "mir_graph.h"
 
 namespace art {