Refine modulo range operations with fixed quotient.

When all combines of x / y produce the same quotient Q, further refine
modulo operations by treating x % y ==  x - Q * y

	PR tree-optimization/125706
	gcc/
	* range-op.cc (operator_trunc_mod::wi_fold): Apply fixed
	quotient refinement.
	gcc/testsuite/
	* gcc.dg/pr125706.c: New.