testsuite: Add testcase for already fixed PR [PR90311]

2020-03-05  Jakub Jelinek  <jakub@redhat.com>

	PR target/90311
	* gcc.c-torture/execute/pr90311.c: New test.

gcc/testsuite/ChangeLog

+2020-03-05  Jakub Jelinek  <jakub@redhat.com>
+
+	PR target/90311
+	* gcc.c-torture/execute/pr90311.c: New test.
+
 2020-03-05  Jeff Law  <law@redhat.com>
 
 	* gcc.target/arm/fuse-caller-save.c: Generalize expected output.

gcc/testsuite/gcc.c-torture/execute/pr90311.c

+/* PR rtl-optimization/90311 */
+
+int a, b;
+
+int
+main ()
+{
+  unsigned long long x;
+  unsigned int c;
+  __builtin_add_overflow ((unsigned char) a, b, &c);
+  b -= c < (unsigned char) a;
+  x = b;
+  if (x)
+    __builtin_abort ();
+  return 0;
+}