use me->def instead of me for opt_table

`vm_opt_method_table` is me=>bop table to manage the optimized
methods (by specialized instruction). However, `me` can be invalidated
to invalidate the method cache entry.
[Bug #17725]

To solve the issue, use `me-def` instead of `me` which simply copied
at invalidation timing.

A test by @jeremyevans #4376

test/ruby/test_method.rb

@@ -1303,6 +1303,21 @@ module M2
     end;
   end
 
+  def test_override_optimized_method_on_class_using_prepend
+    assert_separately(%w(--disable-gems), <<-'end;', timeout: 30)
+      # Bug #17725 [ruby-core:102884]
+      $VERBOSE = nil
+      String.prepend(Module.new)
+      class String
+        def + other
+          'blah blah'
+        end
+      end
+
+      assert_equal('blah blah', 'a' + 'b')
+    end;
+  end
+
   def test_eqq
     assert_operator(0.method(:<), :===, 5)
     assert_not_operator(0.method(:<), :===, -5)

vm.c

@@ -1798,7 +1798,7 @@ rb_iter_break_value(VALUE val)
 
 /* optimization: redefine management */
 
-static st_table *vm_opt_method_table = 0;
+static st_table *vm_opt_method_def_table = 0;
 static st_table *vm_opt_mid_table = 0;
 
 static int
@@ -1852,9 +1852,8 @@ rb_vm_check_redefinition_opt_method(const rb_method_entry_t *me, VALUE klass)
        klass = RBASIC_CLASS(klass);
     }
     if (vm_redefinition_check_method_type(me->def)) {
-	if (st_lookup(vm_opt_method_table, (st_data_t)me, &bop)) {
-	    int flag = vm_redefinition_check_flag(klass);
-
+        if (st_lookup(vm_opt_method_def_table, (st_data_t)me->def, &bop)) {
+            int flag = vm_redefinition_check_flag(klass);
 	    ruby_vm_redefined_flag[bop] |= flag;
 	}
     }
@@ -1885,7 +1884,7 @@ add_opt_method(VALUE klass, ID mid, VALUE bop)
     const rb_method_entry_t *me = rb_method_entry_at(klass, mid);
 
     if (me && vm_redefinition_check_method_type(me->def)) {
-	st_insert(vm_opt_method_table, (st_data_t)me, (st_data_t)bop);
+	st_insert(vm_opt_method_def_table, (st_data_t)me->def, (st_data_t)bop);
 	st_insert(vm_opt_mid_table, (st_data_t)mid, (st_data_t)Qtrue);
     }
     else {
@@ -1899,7 +1898,7 @@ vm_init_redefined_flag(void)
     ID mid;
     VALUE bop;
 
-    vm_opt_method_table = st_init_numtable();
+    vm_opt_method_def_table = st_init_numtable();
     vm_opt_mid_table = st_init_numtable();
 
 #define OP(mid_, bop_) (mid = id##mid_, bop = BOP_##bop_, ruby_vm_redefined_flag[bop] = 0)