[Ada] Simplify detection of a parent interface equality

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

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Replace subtle conditions on First_Entity/Last_Entity with a
straightforward Number_Formals/First_Formal/Last_Formal.

Code cleanup related to handling of dispatching equality in SPARK.

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

gcc/ada/

	* exp_ch3.adb (Predefined_Primitive_Bodies): Simplify detection
	of existing equality operator.

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diff --git a/gcc/ada/exp_ch3.adb b/gcc/ada/exp_ch3.adb
--- a/gcc/ada/exp_ch3.adb
+++ b/gcc/ada/exp_ch3.adb
@@ -10972,16 +10972,13 @@ package body Exp_Ch3 is
          while Present (Prim) loop
             if Chars (Node (Prim)) = Name_Op_Eq
               and then not Is_Internal (Node (Prim))
-              and then Present (First_Entity (Node (Prim)))
 
               --  The predefined equality primitive must have exactly two
-              --  formals whose type is this tagged type
+              --  formals whose type is this tagged type.
 
-              and then Present (Last_Entity (Node (Prim)))
-              and then Next_Entity (First_Entity (Node (Prim)))
-                         = Last_Entity (Node (Prim))
-              and then Etype (First_Entity (Node (Prim))) = Tag_Typ
-              and then Etype (Last_Entity (Node (Prim))) = Tag_Typ
+              and then Number_Formals (Node (Prim)) = 2
+              and then Etype (First_Formal (Node (Prim))) = Tag_Typ
+              and then Etype (Last_Formal (Node (Prim))) = Tag_Typ
             then
                Eq_Needed := False;
                Eq_Name := No_Name;