[Ada] Avoid spurious warning on assertions with Loop_Entry

[Ada] Avoid spurious warning on assertions with Loop_Entry

When the Loop_Entry attribute is used inside a loop invariant or another
assertion where it is allowed, it may lead to spurious warnings on
conditions that are detected to be always valid. Now fixed.

2019-07-11  Yannick Moy  <moy@adacore.com>

gcc/ada/

	* sem_eval.adb (Is_Same_Value): Add special case for rewritten
	Loop_Entry attribute.

gcc/testsuite/

	* gnat.dg/loop_entry1.adb: New testcase.

From-SVN: r273403