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/mir_graph_test.cc b/compiler/dex/mir_graph_test.cc
index 8a7e71f..b3ad040 100644
--- a/compiler/dex/mir_graph_test.cc
+++ b/compiler/dex/mir_graph_test.cc
@@ -14,6 +14,7 @@
  * limitations under the License.
  */
 
+#include "compiler_ir.h"
 #include "mir_graph.h"
 #include "gtest/gtest.h"
 
@@ -148,7 +149,7 @@
 
   TopologicalSortOrderTest()
       : pool_(),
-        cu_(&pool_) {
+        cu_(&pool_, kRuntimeISA, nullptr, nullptr) {
     cu_.mir_graph.reset(new MIRGraph(&cu_, &cu_.arena));
   }