AArch64: Add isnan expander [PR 66462]

AArch64: Add isnan expander [PR 66462]

Add an expander for isnan using integer arithmetic.  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); }

Before:
	fcmp	s0, s0
	cset	w0, vs
	ret

After:
	fmov	w1, s0
	mov	w0, -16777216
	cmp	w0, w1, lsl 1
	cset	w0, cc
	ret

gcc:
	PR middle-end/66462
	* config/aarch64/aarch64.md (isnan<mode>2): Add new expander.

gcc/testsuite:
	PR middle-end/66462
	* gcc.target/aarch64/pr66462.c: Update test.