[Ada] Optimize magnitude of integer operations for fixed point

[Ada] Optimize magnitude of integer operations for fixed point

gcc/ada/

	* exp_fixd.adb (Build_Double_Divide): Use the RM size of types and
	a more precise estimate for the size of the denominator.
	(Build_Double_Divide_Code): Likewise.
	(Build_Multiply): Use a more precise estimate for the size of the
	result.
	(Build_Scaled_Divide):  Use the RM size of types and a more precise
	estimate for the size of the numerator.
	(Build_Scaled_Divide_Code): Likewise.