[Ada] Improve checking for invalid index values when accessing array elements

[Pierre-Marie de Rodat]

Two improvements to the previous change on this topic:

1) Add a guard to prevent a call to Number_Of_Dimensions that would pass
   in a non-array type. This is needed in error cases (see ACATS test
   B95094C).

2) Do not generate the new validity checks in the case where the
   index type in question has a specified Default_Initial_Value aspect
   (which rules out the possibility that an object is invalid because it
   is uninitialized).

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

gcc/ada/

	* exp_ch4.adb (Expand_N_Indexed_Component): The two improvements
	described above.
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