ART: Rewrite core image detection.

Remove unused compiler option `core_image_` and cache
the IsCompilingWithCoreImage in the CompilerOptions instead.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Change-Id: Idcf3b6c96dab9a39f823a16778e7698589442cd8
diff --git a/compiler/driver/compiler_options.h b/compiler/driver/compiler_options.h
index 77f8482..2f4e542 100644
--- a/compiler/driver/compiler_options.h
+++ b/compiler/driver/compiler_options.h
@@ -198,13 +198,6 @@
     return baseline_;
   }
 
-  // Are we compiling a core image (small boot image only used for ART testing)?
-  bool IsCoreImage() const {
-    // Ensure that `core_image_` => `boot_image_`.
-    DCHECK(!core_image_ || boot_image_);
-    return core_image_;
-  }
-
   // Are we compiling an app image?
   bool IsAppImage() const {
     return app_image_;
@@ -214,6 +207,13 @@
     app_image_ = false;
   }
 
+  // Returns whether we are compiling against a "core" image, which
+  // is an indicative we are running tests. The compiler will use that
+  // information for checking invariants.
+  bool CompilingWithCoreImage() const {
+    return compiling_with_core_image_;
+  }
+
   // Should the code be compiled as position independent?
   bool GetCompilePic() const {
     return compile_pic_;
@@ -357,8 +357,8 @@
   HashSet<std::string> image_classes_;
 
   bool boot_image_;
-  bool core_image_;
   bool app_image_;
+  bool compiling_with_core_image_;
   bool baseline_;
   bool debuggable_;
   bool generate_debug_info_;