ART: Add verbose:deopt logging

Add verbose logging for deoptimization. Can be enabled with the
regular command line option "-verbose:_".

Change-Id: Iab63e02f3a4e0a213fc61efed1118fe829feeaa4
diff --git a/runtime/base/logging.h b/runtime/base/logging.h
index 8b34374..35b50d1 100644
--- a/runtime/base/logging.h
+++ b/runtime/base/logging.h
@@ -39,6 +39,7 @@
 struct LogVerbosity {
   bool class_linker;  // Enabled with "-verbose:class".
   bool compiler;
+  bool deopt;
   bool gc;
   bool heap;
   bool jdwp;