Tidy some uses of IsCompilationEnabled.

We should be checking IsCompilationEnabled in the compiler, we wouldn't be in
the compiler were compilation disabled.

Change-Id: Ib498fb6d610e4ed517494df01efa4b64a8c12528
diff --git a/compiler/dex/frontend.cc b/compiler/dex/frontend.cc
index a1e2caa..3f6231c 100644
--- a/compiler/dex/frontend.cc
+++ b/compiler/dex/frontend.cc
@@ -88,9 +88,7 @@
     return nullptr;
   }
 
-  if (!driver.GetCompilerOptions().IsCompilationEnabled()) {
-    return nullptr;
-  }
+  DCHECK(driver.GetCompilerOptions().IsCompilationEnabled());
 
   ClassLinker* class_linker = Runtime::Current()->GetClassLinker();
   CompilationUnit cu(driver.GetArenaPool());