Fix trace exit type for IncDecL of boxed values

For accesses to boxed locals, if the inner type changes, we can
retranslate using HHIR: the new translation will inspect the live type
and be able to execute the instruction.  While looking at something
else, I noticed that in such cases IncDecL was taking a slow exit
(which avoids using HHIR).

hphp/runtime/vm/translator/hopt/hhbctranslator.cpp

@@ -660,7 +660,7 @@ void HhbcTranslator::emitSetL(int32_t id) {
 
 void HhbcTranslator::emitIncDecL(bool pre, bool inc, uint32_t id) {
   // Handle only integer inc/dec for now
-  Trace* exitTrace = getExitSlowTrace();
+  Trace* exitTrace = getExitTrace();
   SSATmp* src = m_tb->genLdLocAsCell(id, exitTrace);
   if (src->isA(Type::Bool)) {
     // inc/dec of a bool is a no-op