ada: Fix weird error in Ada.Numerics.Discrete_Random with Component_Alignment

ada: Fix weird error in Ada.Numerics.Discrete_Random with Component_Alignment

The problem arises when an instance of Ada.Numerics.Discrete_Random is
covered by a pragma Component_Alignment with a non-default alignment.

gcc/ada/ChangeLog:

	* exp_ch5.adb (Expand_Assign_Array): Make Act_Rhs a constant and
	do not recompute it, as well as R_Type, when there is a change of
	representation.  Move comment about the RHS from here to...
	(Expand_N_Assignment_Statement): ...here.