Skip out on processing __builtin_clz when varying.

Skip out on processing __builtin_clz when varying.

The previous changes to irange::constant_p return TRUE for
VARYING, since VARYING has numerical end points like any other
constant range.  The problem is that some users of constant_p
depended on constant_p excluding the full domain.  The
range handler for __builtin_clz, that is shared between ranger
and vr_values, is one such user.

This patch excludes varying_p(), to match the original behavior
for clz.

gcc/ChangeLog:

	PR c/100521
	* gimple-range.cc (range_of_builtin_call): Skip out on
	  processing __builtin_clz when varying.