[Ada] More work on efficiency improvements

[Ada] More work on efficiency improvements

gcc/ada/

	* table.ads (Table_Type): Remove "aliased"; no longer needed by
	Atree.  Besides it contradicted the comment a few lines above,
	"--  Note: We do not make the table components aliased...".
	* types.ads: Move type Slot to Atree.
	* atree.ads: Move type Slot fromt Types to here.  Move type
	Node_Header from Seinfo to here.
	* atree.adb: Avoid the need for aliased components of the Slots
	table.  Instead of 'Access, use a getter and setter.  Misc
	cleanups.
	(Print_Statistics): Print statistics about node and entity kind
	frequencies. Give 3 digit fractions instead of percentages.
	* (Get_Original_Node_Count, Set_Original_Node_Count): Statistics
	for calls to Original_Node and Set_Original_Node.
	(Original_Node, Set_Original_Node): Gather statistics by calling
	the above.
	(Print_Field_Statistics): Print Original_Node statistics.
	(Update_Kind_Statistics): Remove, and put all statistics
	gathering under "if Atree_Statistics_Enabled", which is a flag
	generated in Seinfo by Gen_IL.
	* gen_il-gen.adb (Compute_Field_Offsets): Choose offsets of
	Nkind, Ekind, and Homonym first. This causes a slight efficiency
	improvement.  Misc cleanups.  Do not generate Node_Header; it is
	now hand-written in Atree.  When choosing the order in which to
	assign offsets, weight by the frequency of the node type, so the
	more common nodes get their field offsets assigned earlier.  Add
	more special cases.
	(Compute_Type_Sizes): Remove this and related things.
	There was a comment: "At some point we can instrument Atree to
	print out accurate size statistics, and remove this code." We
	have Atree statistics, so we now remove this code.
	(Put_Seinfo): Generate Atree_Statistics_Enabled, which is equal
	to Statistics_Enabled. This allows Atree to say "if
	Atree_Statistics_Enabled then <gather statistics>" for
	efficiency. When Atree_Statistics_Enabled is False, the "if ..."
	will be optimized away.
	* gen_il-internals.ads (Type_Frequency): New table of kind
	frequencies.
	* gen_il-internals.adb: Minor comment improvement.
	* gen_il-fields.ads: Remove unused subtypes.  Suppress style
	checks in the Type_Frequency table.  If we regenerate this
	table (see -gnatd.A) we don't want to have to fiddle with
	casing.
	* impunit.adb: Minor.
	* sinfo-utils.adb: Minor.
	* debug.adb: Minor comment improvement.