ada: Fix internal error on loop iterator filter with -gnatVa

ada: Fix internal error on loop iterator filter with -gnatVa

The problem is that the condition of the iterator filter is expanded early,
before it is integrated into an if statement of the loop body, so there is
no place to attach the actions generated by this expansion.

This happens only for simple loops, i.e. with a parameter specification, so
the fix uses the same approach for them as for loops based on iterators.

gcc/ada/

	* sinfo.ads (Iterator_Filter): Document field.
	* sem_ch5.adb (Analyze_Iterator_Specification): Move comment around.
	(Analyze_Loop_Parameter_Specification): Only preanalyze the iterator
	filter, if any.
	* exp_ch5.adb (Expand_N_Loop_Statement): Analyze the new list built
	when an iterator filter is present.