tree-optimization/108793 - niter compute type mismatch

When computing the number of iterations until wrap types are mixed up,
eventually leading to checking ICEs with a pointer bitwise inversion.
The following uses niter_type for the calculation.

	PR tree-optimization/108793
	* tree-ssa-loop-niter.cc (number_of_iterations_until_wrap):
	Use convert operands to niter_type when computing num.

	* gcc.dg/torture/pr108793.c: New testcase.