Remove irange::varying_p checks from symbolic_p and constant_p.

Remove irange::varying_p checks from symbolic_p and constant_p.

As of a few releases ago, varying_p() ranges are also constant_p.
Consequently, there is no need to check varying_p from either
symbolic_p or constant_p.

I have adjusted a few users of constant_p that were depending on
constant_p returning false for varying_p.  In these cases, I have
placed the varying_p check before the constant_p check to avoid
the more expensive constant_p check when possible.

gcc/ChangeLog:

	* gimple-ssa-evrp-analyze.c (evrp_range_analyzer::set_ssa_range_info):
	Adjust for constant_p including varying_p.
	* tree-vrp.c (vrp_prop::finalize): Same.
	(determine_value_range): Same.
	* vr-values.c (vr_values::range_of_expr): Same.
	* value-range.cc (irange::symbolic_p): Do not check varying_p.
	(irange::constant_p): Same.