Make irange dependency explicit for range_of_ssa_name_with_loop_info.

Make irange dependency explicit for range_of_ssa_name_with_loop_info.

Even though ranger is type agnostic, SCEV seems to only work with
integers.  This patch removes some FIXME notes making it explicit that
bounds_of_var_in_loop only works with iranges.

Tested on x86-64 Linux.

gcc/ChangeLog:

	* gimple-range-fold.cc (fold_using_range::range_of_phi): Only
	query SCEV for integers.
	(fold_using_range::range_of_ssa_name_with_loop_info): Remove
	irange check.