[dev.regabi] cmd/compile: adjustments to Copy and DeepCopy
DeepCopy is not called DeepSepCopy, so it should use Copy, not SepCopy.
Also, the old gc.treecopy, which became ir.DeepCopy, only copied
the Left, Right, and List fields - not Init, Rlist, Body - and I didn't
notice when I moved it over. A general utility function should of
course copy the whole node, so do that.
Finally, the semantics of Copy should not depend on whether a
particular child node is held directly in a field or in a slice,
so make Copy duplicate the slice backing arrays as well.
(Logically, those backing arrays are part of the node storage.)