Allow copying of symbolic ranges to an irange.

Allow copying of symbolic ranges to an irange.

This fixes an ICE when trying to copy a legacy value_range containing
a symbolic to a multi-range:

	min = make_ssa_name (type);
	max = build_int_cst (type, 55);
	value_range vv (min, max);
	int_range<2> vr = vv;

gcc/ChangeLog:

	* range-op.cc (multi_precision_range_tests): Normalize symbolics when copying to a
	multi-range.
	* value-range.cc (irange::copy_legacy_range): Add test.