[Ada] Optimization loses exception in improper use of 'Value

[Ada] Optimization loses exception in improper use of 'Value

This patch prevents an improper removal of an evaluation of attribute
'Value on an illegal input that will raise Constraint_Error, when a
subsequent use of this evaluation might be optimized away by the
back-end.

2019-07-22  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* libgnat/s-valboo.ads, libgnat/s-valcha.ads,
	libgnat/s-valdec.ads, libgnat/s-valenu.ads,
	libgnat/s-valint.ads, libgnat/s-vallld.ads,
	libgnat/s-vallli.ads, libgnat/s-valllu.ads,
	libgnat/s-valrea.ads, libgnat/s-valuns.ads,
	libgnat/s-valwch.ads: Change categorization of packages that
	implement attribute 'Value from Pure to Preelaborate, to prevent
	undesirable optimizations when the evaluation of the attribute
	raises Constraint_Error, but subsequent use of the result of
	this evsaluation is removed by a subsequent optimization.

gcc/testsuite/

	* gnat.dg/opt80.adb: New testcase.

From-SVN: r273687