coccinelle: add a transformation for GNU conditionals

i.e. x ? x : y => x ?: y

coccinelle/cond-omit-middle.cocci

@@ -0,0 +1,7 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+/* See: https://gcc.gnu.org/onlinedocs/gcc/Conditionals.html#Conditionals */
+@@
+expression e, x;
+@@
+- e ? e : x
++ e ?: x