Add read barriers for the roots in the verifier.

Note: Because the roots (the class references in RegType objects) can
be updated by the read barriers, a lot of uses of type "const RegType"
were replaced with "RegType".

Bug: 12687968
Change-Id: I6cf37a87f352938d43fb51560a8d927ada104f50
diff --git a/runtime/verifier/method_verifier-inl.h b/runtime/verifier/method_verifier-inl.h
index 62ecf4b..d4fe106 100644
--- a/runtime/verifier/method_verifier-inl.h
+++ b/runtime/verifier/method_verifier-inl.h
@@ -66,9 +66,9 @@
   return !failure_messages_.empty();
 }
 
-inline const RegType& MethodVerifier::ResolveCheckedClass(uint32_t class_idx) {
+inline RegType& MethodVerifier::ResolveCheckedClass(uint32_t class_idx) {
   DCHECK(!HasFailures());
-  const RegType& result = ResolveClassAndCheckAccess(class_idx);
+  RegType& result = ResolveClassAndCheckAccess(class_idx);
   DCHECK(!HasFailures());
   return result;
 }