LoongArch: Add isnan expander [PR 66462]

LoongArch: Add isnan expander [PR 66462]

Add an expander for isnan using fclass.  Since isnan is
just a compare, enable it only with -fsignaling-nans to avoid
generating spurious exceptions.  This fixes part of PR66462.

int isnan1 (float x) { return __builtin_isnan (x); }

With -fno-signaling-nans:

	fcmp.cun.s	$fcc0,$f0,$f0
	movcf2fr	$f0,$fcc0
	movfr2gr.s	$r4,$f0
	jr	$r1

With -fsignaling-nans:

	fclass.s	$f0,$f0
	movfr2gr.s	$r4,$f0
	andi	$r4,$r4,3
	sltu	$r4,$r0,$r4
	jr	$r1

	PR middle-end/66462

gcc/

	* config/loongarch/loongarch.md (FCLASS_MASK): Add 3.
	(fclass_optab): Assign isnan for 3.
	(<FCLASS_MASK:fclass_optab><ANYF:mode>2): If FCLASS_MASK is 3,
	only enable when -fsignaling-nans.

gcc/testsuite:

	* gcc.target/loongarch/fclass-compile.c: Update test.
	* gcc.target/loongarch/fclass-run.c: Likewise.