Correct comments for packed shape and index [ci skip]

Author: nobu

vm_callinfo.h

@@ -289,7 +289,7 @@ struct rb_callcache {
 
     union {
         struct {
-          uint64_t value; // Shape ID in upper bits, index in lower bits
+            uint64_t value; // Shape ID in former half, index in latter half
         } attr;
         const enum method_missing_reason method_missing_reason; /* used by method_missing */
         VALUE v;

vm_core.h

@@ -288,7 +288,7 @@ struct iseq_inline_constant_cache {
 };
 
 struct iseq_inline_iv_cache_entry {
-    uint64_t value; // attr_index in lower bits, dest_shape_id in upper bits
+    uint64_t value; // dest_shape_id in former half, attr_index in latter half
     ID iv_set_name;
 };