Add debug counter for ivar inline cache misses that could hit

This commit adds a debug counter for the case where the inline cache
*missed* but the ivar index table has an entry for that ivar.  This is a
case where a polymorphic cache could help

debug_counter.h

@@ -123,6 +123,7 @@ RB_DEBUG_COUNTER(ivar_set_ic_hit)
 RB_DEBUG_COUNTER(ivar_set_ic_miss)
 RB_DEBUG_COUNTER(ivar_set_ic_miss_serial)
 RB_DEBUG_COUNTER(ivar_set_ic_miss_unset)
+RB_DEBUG_COUNTER(ivar_set_ic_miss_iv_hit)
 RB_DEBUG_COUNTER(ivar_set_ic_miss_noobject)
 RB_DEBUG_COUNTER(ivar_get_base)
 RB_DEBUG_COUNTER(ivar_set_base)

vm_insnhelper.c

@@ -1246,6 +1246,7 @@ vm_setivar(VALUE obj, ID id, VALUE val, const rb_iseq_t *iseq, IVC ic, const str
                     ptr = ROBJECT_IVPTR(obj);
                 }
                 RB_OBJ_WRITE(obj, &ptr[index], val);
+                RB_DEBUG_COUNTER_INC(ivar_set_ic_miss_iv_hit);
 
                 return val;
 	    }