ada: Cleanup code for warnings about unset references

ada: Cleanup code for warnings about unset references

Cleanup related to new checks for unset references.

gcc/ada/

	* sem_util.adb
	(In_Pragma_Expression): Add standard guard against searching too
	far.
	(In_Quantified_Expression): Likewise.
	* sem_warn.adb
	(May_Need_Initialized_Actual): Remove redundant parens.
	(Check_References): Remove guard that duplicates a condition from
	the enclosing if-statement; only assign E1T variable when
	necessary.
	(Within_Postcondition): Fix layout.
	(No_Warn_On_In_Out): Balance parens in comment.