[Ada] Extend compile-time evaluation in case statements to all objects

[Pierre-Marie de Rodat]

A comment in Analyze_Case_Statement describes an "interesting
optimization" that was applied to all assignable objects. This patch
extends it to all objects, in particular, to formal parameters of mode
IN.

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

gcc/ada/

        * sem_ch5.adb (Analyze_Case_Statement): Extend optimization to
        all objects; fix typo in comment.
diff --git a/gcc/ada/sem_ch5.adb b/gcc/ada/sem_ch5.adb
--- a/gcc/ada/sem_ch5.adb
+++ b/gcc/ada/sem_ch5.adb
@@ -1456,7 +1456,7 @@ package body Sem_Ch5 is
          if Is_Entity_Name (Exp) then
             Ent := Entity (Exp);
 
-            if Is_Assignable (Ent) then
+            if Is_Object (Ent) then
                if List_Length (Choices) = 1
                  and then Nkind (First (Choices)) in N_Subexpr
                  and then Compile_Time_Known_Value (First (Choices))
@@ -1475,7 +1475,7 @@ package body Sem_Ch5 is
             end if;
          end if;
 
-         --  Case where expression is not an entity name of a variable
+         --  Case where expression is not an entity name of an object
 
          Analyze_Statements (Statements (Alternative));
       end Process_Statements;