[Ada] Simplify redundant checks for non-empty lists

[Pierre-Marie de Rodat]

Simplify "Present (L) and then not Is_Empty_List (L)" into "not
Is_Empty_List (L)", since Is_Empty_List can be called on No_List
and returns True.

Code cleanup; semantics is unaffected.

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

gcc/ada/

	* sem_ch12.adb, sem_ch6.adb, sem_ch9.adb, sprint.adb: Simplify
	checks for non-empty lists.
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