[dev.regabi] cmd/compile: move gc.treecopy to ir.DeepCopy
This is a general operation on IR nodes, so it belongs in ir.
The copied implementation is adapted to support the
extension pattern, allowing nodes to implement their
own DeepCopy implementations if needed.
This is the first step toward higher-level operations instead
of Left, Right, etc. It will allow the new type syntax nodes
to be properly immutable and opt out of those fine-grained methods.