ada: Exception-raising loop incorrectly eliminated

If the body of a loop includes a raise statement then the loop should not be
considered to be free of side-effects and therefore eligible for elimination
by the compiler.

gcc/ada/ChangeLog:

	* sem_util.adb
	(Side_Effect_Free_Statements): Return False if the statement list
	includes an explicit (i.e. Comes_From_Source) raise statement.