maxval on -inf and nan in Fortran

[Jiufu Guo]

Hi,

When I check a PR93709, I find the testcase maxlocval_4.f90 and
minlocval_4.f90 are checking `maxval/minval` on `-inf` and `nan`.
https://gcc.gnu.org/git/gitweb.cgi?p=gcc.git;a=blob_plain;f=gcc/testsuite/gfortran.dg/maxlocval_4.f90;hb=HEAD
There are code like:
```
l5 = .true.
l5(1,1) = .false.
l5(1,2) = .false.
l5(2,3) = .false.
a = reshape ((/ nan, nan, nan, minf, minf, minf, minf, pinf, minf /), (/ 3, 3 /))
b = maxval (a, dim = 1)
if (.not.isnan(b(1))) STOP 3
a = nan
a(1,3) = minf
if (maxval (a).ne.minf) STOP 65
if (maxval (a, mask = l5).ne.minf) STOP 70
```

While we know, as gfortran manual said at
5.4 MAX and MIN intrinsics with REAL NaN arguments:
https://gcc.gnu.org/onlinedocs/gcc-9.2.0/gfortran/MAX-and-MIN-intrinsics-with-REAL-NaN-arguments.html#MAX-and-MIN-intrinsics-with-REAL-NaN-arguments

```
The Fortran standard does not specify what the result of the MAX and MIN
intrinsics are if one of the arguments is a NaN.  Accordingly, the GNU
Fortran compiler does not specify that either, as this allows for faster
and more compact code to be generated.  If the programmer wishes to take
some specific action in case one of the arguments is a NaN, it is necessary
to explicitly test the arguments before calling MAX or MIN, e.g. with the
IEEE_IS_NAN function from the intrinsic module IEEE_ARITHMETIC.
```

The test case does not check NaN explicitly.  So, strictly speaking,
this code may need more stronger to check NaN, otherwise it may STOP
during execution, and this STOP is acceptable. Right?

With -ffast-math -O3, this case `STOP 3` on a few platforms, e.g. ppc64le/x86.


Thanks,

Jiufu