s390: Optimize fmin/fmax.

s390: Optimize fmin/fmax.

On VXE targets, we can directly use the fp min/max instruction instead of
calling into libm for fmin/fmax etc.

Provide fmin/fmax versions also for vectors even though it cannot be
called directly.  This will be exploited with a follow-up patch when
reductions are introduced.

gcc/ChangeLog:

	* config/s390/s390.md: Update UNSPECs
	* config/s390/vector.md (fmax<mode>3): New expander.
	(fmin<mode>3): New expander.
	* config/s390/vx-builtins.md (*fmin<mode>): New insn.
	(vfmin<mode>): Redefined to use new insn.
	(*fmax<mode>): New insn.
	(vfmax<mode>): Redefined to use new insn.

gcc/testsuite/ChangeLog:

	* gcc.target/s390/fminmax-1.c: New test.
	* gcc.target/s390/fminmax-2.c: New test.

Signed-off-by: Juergen Christ <jchrist@linux.ibm.com>