[frange] Avoid testing signed zero test for -fno-signed-zeros.

This patch moves a test that is meant to only work for signed zeros
into range_tests_signed_zeros.

I am not aware of any architectures where this is failing, but it is
annoying to see selftests failing when -fno-signed-zeros is used.

gcc/ChangeLog:

	* value-range.cc (range_tests_signbit): Move to set from here...
	(range_tests_signed_zeros): ...to here.