[Ada] Follow-on efficiency improvements

[Pierre-Marie de Rodat <derodat@adacore.com>]

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This patch improves compiler efficiency by placing the Homonym attribute
at offset zero. Homonym is a heavily used field.

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

gcc/ada/

	* gen_il-gen.adb: Set the number of concrete nodes that have the
	Homonym field to a higher number than any other field. This
	isn't true, but it forces Homonym's offset to be chosen first,
	so it will be at offset zero and hence slot zero.

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diff --git a/gcc/ada/gen_il-gen.adb b/gcc/ada/gen_il-gen.adb
--- a/gcc/ada/gen_il-gen.adb
+++ b/gcc/ada/gen_il-gen.adb
@@ -1036,6 +1036,13 @@ package body Gen_IL.Gen is
             Append (All_Fields, F);
          end loop;
 
+         --  Force Homonym to be at offset zero, which speeds up the
+         --  compiler. The Sort below will place Homonym first in
+         --  All_Fields.
+
+         Num_Concrete_Have_Field (Homonym) :=
+           Num_Concrete_Have_Field (Nkind) + 1;
+
          --  Sort All_Fields based on how many concrete types have the field.
          --  This is for efficiency; we want to choose the offsets of the most
          --  common fields first, so they get low numbers.