IBM Z: Prevent mach optimization on doloop patterns

IBM Z: Prevent mach optimization on doloop patterns

s390_fix_long_loop_prediction is used in machine dependent
reorg. There we use single_set to distingiush between conditional
jumps and branch on count patterns.  However, single_set returns a
non-NULL value also for PARALLELs in case one of the SETs is dead.

2020-06-17  Andreas Krebbel  <krebbel@linux.ibm.com>

gcc/
	* config/s390/s390.c (s390_fix_long_loop_prediction): Exit early
	for PARALLELs.

gcc/testsuite/
	* gcc.target/s390/20200617.c: New test.