ada: Cleanup validity of boolean operators

ada: Cleanup validity of boolean operators

Move detection of always valid expressions from routine
Ensure_Valid (which inserts validity checks) to Expr_Known_Valid
(which decides their validity). In particular, this patch removes
duplicated detection of boolean operators, which were recognized
in both these routines.

Code cleanup; behavior is unaffected.

gcc/ada/

	* checks.adb (Ensure_Valid): Remove detection of boolean and
	short-circuit operators.
	(Expr_Known_Valid): Detect short-circuit operators; detection of
	boolean operators was already done in this routine.