[dev.regabi] cmd/compile: replace ir.Name map with ir.NameSet for SSA 2

Same as CL 284897, the last one.

Passes toolstash -cmp.

Updates #43819

Change-Id: I0bd8958b3717fb58a5a6576f1819a85f33b76e2d
Reviewed-on: https://go-review.googlesource.com/c/go/+/285913


Run-TryBot: Baokun Lee <bk@golangcn.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Trust: Baokun Lee <bk@golangcn.org>