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/quick/mips/codegen_mips.h b/compiler/dex/quick/mips/codegen_mips.h
index ac14704..a37fe40 100644
--- a/compiler/dex/quick/mips/codegen_mips.h
+++ b/compiler/dex/quick/mips/codegen_mips.h
@@ -17,12 +17,13 @@
 #ifndef ART_COMPILER_DEX_QUICK_MIPS_CODEGEN_MIPS_H_
 #define ART_COMPILER_DEX_QUICK_MIPS_CODEGEN_MIPS_H_
 
-#include "dex/compiler_internals.h"
 #include "dex/quick/mir_to_lir.h"
 #include "mips_lir.h"
 
 namespace art {
 
+struct CompilationUnit;
+
 class MipsMir2Lir FINAL : public Mir2Lir {
  protected:
   class InToRegStorageMipsMapper : public InToRegStorageMapper {