[Ada] Refine type in expansion of array equality

[Pierre-Marie de Rodat]

Code cleanup related to expansion of predefined equality for GNATprove;
semantics is unaffected.

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

gcc/ada/

	* exp_ch4.adb (Handle_One_Dimension): Parameter N must be always
	positive, because it is translated into expression of 'First and
	'Last.
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