[Ada] Optimize 2**N in more contexts

[Arnaud Charlet <charlet at adacore dot com>]

This patch optimizes 2**N to a shift in more contexts. Previously this
optimization was applied only when 2**N appeared as the operand of a
multiply or divide. Now it applies in other contexts as well. The
following test program:

function Pow2_Shift
  (B : Integer;
   N : Natural) return Natural
is
begin
   if B = 0 then
      return 2 ** N;
   elsif B = 1 then
      return 4 * 2 ** N;
   else
      return 1 * 2 ** N;
   end if;
end Pow2_Shift;

generates the following output with -gnatG:

function pow2_shift (b : integer; n : natural) return natural is
begin
   if b = 0 then
      [constraint_error when
        not (shift_left!(1, n) >= 0)
        "range check failed"]
      return natural(shift_left!(1, n));
   elsif b = 1 then
      [constraint_error when
        not (shift_left!(4, n) >= 0)
        "range check failed"]
      return natural(shift_left!(4, n));
   else
      [constraint_error when
        not (shift_left!(1, n) >= 0)
        "range check failed"]
      return natural(shift_left!(1, n));
   end if;
end pow2_shift;

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

2010-06-14  Robert Dewar  <dewar@adacore.com>

	* exp_ch4.adb (Expand_N_Op_Expon): Optimize 2**N in stand alone context

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