Add DataFlowGraph::redefine_first_value()
Add DataFlowGraph::redefine_first_value()

This makes it possible to compute the first result of an instruction in
a different way without overwriting the original instruction with
replace().

stoklund/cretonne,sunfishcode/cretonne,sunfishcode/cretonne,stoklund/cretonne,sunfishcode/cretonne,stoklund/cretonne