[dev.regabi] cmd/compile: make copyExpr return *ir.Name directly

copyExpr just calls copyExpr1 with "clear" is false, so make it return
*ir.Name directly instead of ir.Node

Passes toolstash -cmp.

Change-Id: I31ca1d88d9eaf8ac37517022f1c74285ffce07d3
Reviewed-on: https://go-review.googlesource.com/c/go/+/280714


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>