rs6000: Check -+0 and NaN for smax/smin generation

rs6000: Check -+0 and NaN for smax/smin generation

PR93709 mentioned regressions on maxlocval_4.f90 and minlocval_f.f90 which
relates to max of '-inf' and 'nan'. This regression occur on P9 because
P9 new instruction 'xsmaxcdp' is generated.
And for C code `a < b ? b : a` is also generated as `xsmaxcdp` under -O2
for P9. While this instruction behavior more like C/C++ semantic (a>b?a:b).

This generates prevents 'xsmaxcdp' to be generated for those cases.
'xsmincdp' also is handled in patch.

gcc/
2020-03-10  Jiufu Guo  <guojiufu@linux.ibm.com>

	PR target/93709
	* gcc/config/rs6000/rs6000.c (rs6000_emit_p9_fp_minmax): Check
	NAN and SIGNED_ZEROR for smax/smin.

gcc/testsuite
2020-03-10  Jiufu Guo  <guojiufu@linux.ibm.com>

	PR target/93709
	* gcc.target/powerpc/p9-minmax-3.c: New test.