s390/testsuite: Fix oscbreak-1.c.

Checking for an osc break is somewhat brittle especially with many
passes potentially introducing new insns and moving them around.
Therefore, only compile the test with -O1 -fschedule-insns in order
to limit the influence of other passes.

gcc/testsuite/ChangeLog:

	* gcc.target/s390/oscbreak-1.c: Compile with -O1
	-fschedule-insns

gcc/testsuite/gcc.target/s390/oscbreak-1.c

 /* { dg-do compile } */
-/* { dg-options "-O3 -mtune=z13 -dp" } */
+/* { dg-options "-O1 -mtune=z13 -fschedule-insns -dp" } */
 
 void
 foo (char *a, int b)
@@ -10,6 +10,7 @@ foo (char *a, int b)
     a[i] += 1;
 }
 
+/* We should not unconditionally emit an osc_break here.  */
 void
 bar (char *a, int b)
 {
@@ -22,4 +23,4 @@ bar (char *a, int b)
     }
 }
 
-/* { dg-final { scan-assembler "osc_break" } } */
+/* { dg-final { scan-assembler-times "osc_break" 1 } } */