Inhibit constant propagation inlining SYMBOL_REF loads into
UNSPECV_CAS.  Even though reload can later undo it, the resulting
code will be less efficient.

gcc/ChangeLog:

2018-09-06  Ilya Leoshkevich  <iii@linux.ibm.com>

	PR target/80080
	* config/s390/predicates.md: Add nonsym_memory_operand.
	* config/s390/s390.c (s390_legitimize_cs_operand): If operand
	contains a SYMBOL_REF, load it into an intermediate pseudo.
	(s390_emit_compare_and_swap): Legitimize operand.
	* config/s390/s390.md: Use the new nonsym_memory_operand
	with UNSPECV_CAS patterns.

gcc/testsuite/ChangeLog:

2018-09-06  Ilya Leoshkevich  <iii@linux.ibm.com>

	PR target/80080
	* gcc.target/s390/pr80080-3.c: New test.
	* gcc.target/s390/s390.exp: Make sure the new test passes
	on all optimization levels.

From-SVN: r264143