[gdb] Add template functions assign_return/set_if_changed

Add template functions assign_return_if_changed and assign_set_if_changed in
gdb/utils.h:
...
template<typename T> void assign_set_if_changed (T &lval, const T &val, bool &changed)
{ ... }
template<typename T> bool assign_return_if_changed (T &lval, const T &val)
{ ... }
...

This allows us to rewrite code like this:
...
  if (tui_border_attrs != entry->value)
    {
      tui_border_attrs = entry->value;
      need_redraw = true;
    }
...
into this:
...
  need_redraw |= assign_return_if_changed<int> (tui_border_attrs, entry->value);
...
or:
...
  assign_set_if_changed<int> (tui_border_attrs, entry->value, need_redraw);
...

The names are a composition of the functionality.  The functions:
- assign VAL to LVAL, and either
- return true if the assignment changed LVAL, or
- set CHANGED to true if the assignment changed LVAL.

Tested on x86_64-linux.

gdb/utils.c

@@ -3631,6 +3631,43 @@ copy_bitwise (gdb_byte *dest, ULONGEST dest_offset,
     }
 }
 
+#if GDB_SELF_TEST
+static void
+test_assign_set_return_if_changed ()
+{
+  bool changed;
+  int a;
+
+  for (bool initial : { false, true })
+    {
+      changed = initial;
+      a = 1;
+      assign_set_if_changed (a, 1, changed);
+      SELF_CHECK (a == 1);
+      SELF_CHECK (changed == initial);
+    }
+
+  for (bool initial : { false, true })
+    {
+      changed = initial;
+      a = 1;
+      assign_set_if_changed (a, 2, changed);
+      SELF_CHECK (a == 2);
+      SELF_CHECK (changed == true);
+    }
+
+  a = 1;
+  changed = assign_return_if_changed (a, 1);
+  SELF_CHECK (a == 1);
+  SELF_CHECK (changed == false);
+
+  a = 1;
+  assign_set_if_changed (a, 2, changed);
+  SELF_CHECK (a == 2);
+  SELF_CHECK (changed == true);
+}
+#endif
+
 void _initialize_utils ();
 void
 _initialize_utils ()
@@ -3695,5 +3732,7 @@ When set, debugging messages will be marked with seconds and microseconds."),
   selftests::register_test ("strncmp_iw_with_mode",
 			    strncmp_iw_with_mode_tests);
   selftests::register_test ("pager", test_pager);
+  selftests::register_test ("assign_set_return_if_changed",
+			    test_assign_set_return_if_changed);
 #endif
 }

gdb/utils.h

@@ -337,4 +337,31 @@ extern void copy_bitwise (gdb_byte *dest, ULONGEST dest_offset,
 
 extern int readline_hidden_cols;
 
+/* Assign VAL to LVAL, and set CHANGED to true if the assignment changed
+   LVAL.  */
+
+template<typename T>
+void
+assign_set_if_changed (T &lval, const T &val, bool &changed)
+{
+  if (lval == val)
+    return;
+
+  lval = val;
+  changed = true;
+}
+
+/* Assign VAL to LVAL, and return true if the assignment changed LVAL.  */
+
+template<typename T>
+bool
+assign_return_if_changed (T &lval, const T &val)
+{
+  if (lval == val)
+    return false;
+
+  lval = val;
+  return true;
+}
+
 #endif /* UTILS_H */