Bump isl for special casing intersection of identical sets

Sven Verdoolaege · ftynse · commit be028d4042a0 · 2018-06-15T11:12:26.000+02:00
diff --git a/third-party/islpp b/third-party/islpp
@@ -1 +1 @@
-Subproject commit 0cf707e5003570c9df45a0b5b776429b5f0ce298
+Subproject commit 9f13e434467b7a5118cf54f335f3aef7ce6dc76d
