Allow single PHI initial values.

Allow single PHI initial values.

There are some single PHI groups that can benefit from an initial
value.  Also improve the iteration calculation by bounding each
iteration with the known global value.

	PR tree-optimization/121345
	gcc/
	* gimple-range-phi.cc (phi_group::phi_group): Add modifier name.
	(phi_group::is_modifier_p): Set modifier stmt operand name.
	(phi_group::calculate_using_modifier): Bound the iteration range
	by known global range.
	(phi_analyzer::process_phi): Allow single PHIS if they meet certain
	criteria.
	* gimple-range-phi.h (m_modifier_name): New member.
	(is_modifier_p): Adjust prototype.

	gcc/testsuite/
	* g++.dg/pr121345.C: New.