Fix uninitialized variable in Atree.Size_In_Slots

Fix uninitialized variable in Atree.Size_In_Slots

Size_In_Slots uses the Nkind to look up the size in a table indexed
by Nkind.  This patch fixes a couple of places where the Nkind is
wrong (uninitialized or zeroed out) so Size_In_Slots cannot be used.

gcc/ada/
	PR ada/100564
	* atree.adb (Change_Node): Do not call Zero_Slots on a Node_Id
	when the Nkind has not yet been set; call the other Zero_Slots
	that takes a range of slot offsets.  Call the new Mutate_Kind
	that takes an Old_Size, for the same reason -- the size cannot
	be computed without the Nkind.
	(Mutate_Nkind): New function that allows specifying the Old_Size.
	(Size_In_Slots): Assert that the Nkind has proper (nonzero) value.
	* atree.ads: Minor reformatting.