[Ada] Optimize generation of checks for fixed-point types

[Ada] Optimize generation of checks for fixed-point types

gcc/ada/

	* checks.ads (Determine_Range_To_Discrete): New procedure.
	* checks.adb (Apply_Scalar_Range_Check): Call it to determine
	a range for the expression when the target type is discrete.
	And also apply the tests for discrete types to fixed-point
	types when they are treated as integers.
	(Apply_Type_Conversion_Checks): Apply checks to conversions
	involving fixed-point types when they are treated as integers.
	(Determine_Range) <N_Type_Conversion>: Factor out code into...
	(Determine_Range_To_Discrete): ...this new procedure and add
	support for fixed-point types when they are treated as integers.
	* einfo.ads (Type_High_Bound): Remove obsolete sentence.
	(Type_Low_Bound): Likewise.
	* exp_ch4.adb (Discrete_Range_Check): Remove obsolete code.
	(Real_Range_Check): Likewise.
	(Expand_N_Type_Conversion): In case of a no-op conversion, clear
	the Do_Range_Check flag on the operand before substituting it.
	Remove calls to Real_Range_Check and Discrete_Range_Check that
	are not guarded by the Do_Range_Check flag, and an assertion.
	* sem_res.adb (Resolve_Type_Conversion): Always apply range
	checks in GNATprove mode; in normal mode, use the updated type
	of the operand in the test against Universal_Fixed.  Remove
	obsolete code setting the Do_Range_Check flag at the end.