[Ada] Optization of predicate checks

[Arnaud Charlet <charlet at adacore dot com>]

This patch marks the generated predicate functions as Pure, so that the
back-end can optimize redundant calls to these functions when inlining and
high level of optimization are requested.

This is a performance enhancement, no change in behavior.

Tested on x86_64-pc-linux-gnu, committed on trunk

2015-10-16  Ed Schonberg  <schonberg@adacore.com>

	* sem_ch13.adb (Build_Predicate_Functions): If expression for
	predicate is side-effect free, indicate that the predicate
	function is pure, to allow for optimization of redundant
	predicate checks.

Attachment: difs
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