[gfortran,patch] Make MIN and MAX handle NaNs correctly (PR32048)

[FX Coudert]

Attached patch makes MIN and MAX intrinsics return correct IEEE  
results (and consistent results) when given one or more NaN as  
arguments. It does so by changing the code generated from MAX(a,b) :=  
(b > a ? b : a) into MAX(a,b) := (b > a || isnan(a) ? b : a);  
handling of multiple arguments is done similarly.

The introduction of isnan() tests probably has a cost, but hey, we  
want a correct result anyway, don't we? Thus, I added a note into the  
code saying that when IEEE_ARITHMETIC is implemented, we might want  
to make this codepath dependent on IEEE_ARITHMETIC being used (I  
don't think we should do that, but we should certainly discuss the  
option).

I also added a testcase, I don't have much experience with NaNs so I  
don't know if all targets have them, does someone know if "x = 0.0 ;  
x = x / x;" will result in a NaN in all cases?

Bootstrapped and regtested on x86_64-linux, OK for mainline?
FX

:ADDPATH fortran:
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