add example of bug

src/test/resources/regressions/features/overflow_checks/overflow_simple1.go

@@ -0,0 +1,17 @@
+// Any copyright is dedicated to the Public Domain.
+// http://creativecommons.org/publicdomain/zero/1.0/
+
+package main
+
+// ##(--overflow)
+
+const MinInt64 = -9223372036854775808 // = -1 << 63
+// @ requires x > MinInt64
+// @ ensures res >= 0
+func abs(x int64) (res int64) {
+	if x < 0 {
+		return -x
+	} else {
+		return x
+	}
+}