ada: Don't reuse operator nodes in expansion

ada: Don't reuse operator nodes in expansion

This patch removes handling of references to unset objects that relied
on Original_Node. This handling was only needed because of rewriting
that reused operator nodes, for example, when an array inequality like:

  A < B

was rewritten into:

  System.Compare_Array_Unsigned_8.Compare_Array_U8
    (A'Address, B'Address, A'Length, B'Length) < 0

by keeping the node for operator "<" and only substituting its operands.
It seems safer to simply create an new operator node when rewriting and
not rely on Original_Node afterwards.

Cleanup related to improved detection uninitialized objects.

gcc/ada/

	* checks.adb (Apply_Arithmetic_Overflow_Strict): Rewrite using a
	newly created operator node.
	* exp_ch4.adb (Expand_Array_Comparison): Likewise.
	* exp_ch6.adb (Add_Call_By_Copy_Code): Rewriting actual parameter
	using its own location and not the location of the subprogram
	call.
	* sem_warn.adb (Check_References): Looping with Original_Node is
	no longer needed.