LoongArch: Use UNSPEC for fmin/fmax RTL pattern [PR105414]

LoongArch: Use UNSPEC for fmin/fmax RTL pattern [PR105414]

I made a mistake defining fmin/fmax RTL patterns in r13-2085: I used
smin and smax in the definition mistakenly.  This causes the optimizer
to perform constant folding as if fmin/fmax was "really" smin/smax
operations even with -fsignaling-nans.  Then pr105414.c fails.

We don't have fmin/fmax RTL codes for now (PR107013) so we can only use
an UNSPEC for fmin and fmax patterns.

gcc/ChangeLog:

	PR tree-optimization/105414
	* config/loongarch/loongarch.md (UNSPEC_FMAX): New unspec.
	(UNSPEC_FMIN): Likewise.
	(fmax<mode>3): Use UNSPEC_FMAX instead of smax.
	(fmin<mode>3): Use UNSPEC_FMIN instead of smin.