[Ada] Simplify expansion of conditional expressions

[Arnaud Charlet <charlet at adacore dot com>]

This patch does two things. First, if Use_Expressions_With_Actions is
set, then the normal case expansion of conditional actions is simplified
by using this node. Second if Back_End_Handles_Limited_Types is set,
then the special case expansion for limited types is eliminated.

The following test:

function EA_Cond_Expr
  (a, b, c : Integer; s : String) return Boolean
is
begin
   return (if b <= c then s (a .. b) = s (b .. c)
                     else s (a .. b) = s (c .. b));
end;

compiled with -gnatd.X to force use of Expression_With_Actions
(and also -gnatX -gnatpG) generates the following -gnatG output:

function ea_cond_expr (a : integer; b : integer; c : integer; s :
  string) return boolean is
   subtype ea_cond_expr__S1b is string (s'first(1) .. s'last(1));
begin
   return (if b <= c then
      do
         [subtype ea_cond_expr__T3b is integer range a .. b]
         [subtype ea_cond_expr__T4b is string (ea_cond_expr__T3b)]
         reference ea_cond_expr__T4b
         [subtype ea_cond_expr__T5b is integer range b .. c]
         [subtype ea_cond_expr__T6b is string (ea_cond_expr__T5b)]
         reference ea_cond_expr__T6b
      in s (a .. b) = s (b .. c) end
    else
      do
         [subtype ea_cond_expr__T7b is integer range a .. b]
         [subtype ea_cond_expr__T8b is string (ea_cond_expr__T7b)]
         reference ea_cond_expr__T8b
         [subtype ea_cond_expr__T9b is integer range c .. b]
         [subtype ea_cond_expr__T10b is string (ea_cond_expr__T9b)]
         reference ea_cond_expr__T10b
      in s (a .. b) = s (c .. b) end
   );
end ea_cond_expr;

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

2010-06-18  Robert Dewar  <dewar@adacore.com>

	* debug.adb: New debug flag -gnatd.L to control
	Back_End_Handles_Limited_Types.
	* exp_ch4.adb (Expand_N_Conditional_Expression): Let back end handle
	limited case if Back_End_Handles_Limited_Types is True.
	(Expand_N_Conditional_Expression): Use N_Expression_With_Actions to
	simplify expansion if Use_Expression_With_Actions is True.
	* gnat1drv.adb (Adjust_Global_Switches): Set
	Back_End_Handles_Limited_Types.
	* opt.ads (Back_End_Handles_Limited_Types): New flag.

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