Clear NAN when reading back a global range if necessary.

When reading back from the global store, we must clear the NAN bit if
necessary.  The reason it's not happening is because the constructor
sets a NAN by default (when HONOR_NANS).  We must be careful to clear
the NAN bit if the original range didn't have a NAN.

I have commented the reason we use the constructor instead of filling
out the fields by hand, because it wasn't clear at re-reading this
code.

	PR 107569/tree-optimization

gcc/ChangeLog:

	* value-range-storage.cc (frange_storage_slot::get_frange): Clear
	NAN if appropriate.
	* value-range.cc (range_tests_floats): New test.