[Ada] Small cleanup throughout Exp_Ch9

[Pierre-Marie de Rodat]

This mostly replaces verbose lines with calls to the Entry_Index_Type
accessor functions and also changes Collect_Entry_Families to using
the same method as Build_Entry_Count_Expression to determine whether
an entry family is large.

No functional changes.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* exp_ch9.adb (Is_Potentially_Large_Family): Add documentation.
	(Actual_Index_Expression): Use Entry_Index_Type.
	(Build_Entry_Count_Expression): Likewise.
	(Build_Find_Body_Index): Likewise.
	(Collect_Entry_Families): Likewise. Use directly the bounds of
	the index type to find out whether the family is large.
	(Entry_Index_Expression): Likewise.
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