ada: Small fix to Default_Initialize_Object

Unlike what is assumed in other parts of the front-end, some objects created
with No_Initialization set on their declaration may end up being initialized
with a default value.

gcc/ada/

	* exp_ch3.adb (Default_Initialize_Object): Return immediately when
	either Has_Init_Expression or No_Initialization is set on the node.
	Tidy up the rest of the code accordingly.
	(Simple_Initialization_OK): Do not test Has_Init_Expression here.