Update isl to c3892bebc0

Various smaller improvements and bugfixes.

llvm-svn: 236932
diff --git a/polly/lib/External/isl/isl_map.c b/polly/lib/External/isl/isl_map.c
index 7bc6d14..6cfc1da 100644
--- a/polly/lib/External/isl/isl_map.c
+++ b/polly/lib/External/isl/isl_map.c
@@ -19,6 +19,7 @@
 #include <isl_ctx_private.h>
 #include <isl_map_private.h>
 #include <isl_blk.h>
+#include <isl/constraint.h>
 #include "isl_space_private.h"
 #include "isl_equalities.h"
 #include <isl_lp_private.h>
@@ -9066,6 +9067,9 @@
 	int i, cmp;
 	unsigned total;
 
+	if (!bmap1 || !bmap2)
+		return -1;
+
 	if (bmap1 == bmap2)
 		return 0;
 	if (ISL_F_ISSET(bmap1, ISL_BASIC_MAP_RATIONAL) !=