[frange] Convert operator_negate::fold_range to sub-ranges.

Negate each sub-range on its own and union the results, so the gaps
between sub-ranges survive instead of collapsing to the [lower, upper]
hull.

Tested on ppc64le: regstrap, LAPACK.

gcc/ChangeLog:

	* range-op-float.cc (operator_negate::fold_range): Negate each
	sub-range and union the results.
	(range_op_float_tests): New test.