tree-optimization/115701 - factor out maybe_duplicate_ssa_info_at_copy

tree-optimization/115701 - factor out maybe_duplicate_ssa_info_at_copy

The following factors out the code that preserves SSA info of the LHS
of a SSA copy LHS = RHS when LHS is about to be eliminated to RHS.

	PR tree-optimization/115701
	* tree-ssanames.h (maybe_duplicate_ssa_info_at_copy): Declare.
	* tree-ssanames.cc (maybe_duplicate_ssa_info_at_copy): New
	function, split out from ...
	* tree-ssa-copy.cc (fini_copy_prop): ... here.
	* tree-ssa-sccvn.cc (eliminate_dom_walker::eliminate_stmt): ...
	and here.