[dev.regabi] cmd/compile: change LocalSlot.N to *ir.Name

[dev.regabi] cmd/compile: change LocalSlot.N to *ir.Name

This was already documented as always being an ONAME, so it just
needed a few type assertion changes.

Passes buildall w/ toolstash -cmp.

Updates #42982.

Change-Id: I61f4b6ebd57c43b41977f4b37b81fe94fb11a723
Reviewed-on: https://go-review.googlesource.com/c/go/+/275757
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>