[dev.regabi] cmd/compile: replace many uses of ir.Node with *ir.Name

[dev.regabi] cmd/compile: replace many uses of ir.Node with *ir.Name

This commit adds exactly two "n := n.(*ir.Name)" statements, that are
each immediately preceded by a "case ir.ONAME:" clause in an n.Op()
switch. The rest of the changes are simply replacing "ir.Node" to
"*ir.Name" and removing now unnecessary "n.(*ir.Name)" type
assertions, exposing the latent typing details.

Passes buildall w/ toolstash -cmp.

Updates #42982.

Change-Id: I8ea3bbb7ddf0c7192245cafa49a19c0e7a556a39
Reviewed-on: https://go-review.googlesource.com/c/go/+/275791
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Russ Cox <rsc@golang.org>