LoongArch: Implement scalar isinf, isnormal, and isfinite via fclass

LoongArch: Implement scalar isinf, isnormal, and isfinite via fclass

Doing so can avoid loading FP constants from the memory.  It also
partially fixes PR 66262 as fclass does not signal on sNaN.

gcc/ChangeLog:

	* config/loongarch/loongarch.md (extendsidi2): Add ("=r", "f")
	alternative and use movfr2gr.s for it.  The spec clearly states
	movfr2gr.s sign extends the value to GRLEN.
	(fclass_<fmt>): Make the result SImode instead of a floating
	mode.  The fclass results are really not FP values.
	(FCLASS_MASK): New define_int_iterator.
	(fclass_optab): New define_int_attr.
	(<FCLASS_MASK:fclass_optab><ANYF:mode>): New define_expand
	template.

gcc/testsuite/ChangeLog:

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