ada: Optimize 2**N to avoid explicit 'if' in modular case

ada: Optimize 2**N to avoid explicit 'if' in modular case

The compiler usually turns 2**N into Shift_Left(1,N).
This patch removes the check for "shift amount too big" in the
modular case, because Shift_Left works properly in that case
(i.e. if N is very large, it returns 0).

This removes a redundant check on most hardware; Shift_Left
takes care of large shirt amounts as necessary, even though
most hardware does not.

gcc/ada/

	* exp_ch4.adb
	(Expand_N_Op_Expon): Remove the too-big check. Simplify. Signed
	and modular cases are combined, etc. Remove code with comment "We
	only handle cases where the right type is a[sic] integer", because
	the right operand must always be an integer at this point.