[Ada] Skip unneeded expansion for Alfa mode

[Arnaud Charlet <charlet at adacore dot com>]

In Alfa mode for formal verification, a special expansion done in the frontend
turns out to be both harmful and unneeded, because the formal verification
backend relies on the types of nodes (hence is not robust w.r.t. a change to
base type here), and does not suffer from the out-of-order issue targetted by
the special expansion. Thus, this expansion is skipped in Alfa mode.

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

2012-08-06  Yannick Moy  <moy@adacore.com>

	* sem_attr.adb (Analyze_Attribute): In the case for 'Old,
	skip a special expansion which is not needed in Alfa mode.

Attachment: difs
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