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

Same as CL 284897, but for SSA.

Passes toolstash -cmp.

Updates #43819

Change-Id: I3c500ad635a3192d95d16fdc36f154ba3ea5df69
Reviewed-on: https://go-review.googlesource.com/c/go/+/284898


Run-TryBot: Baokun Lee <bk@golangcn.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Baokun Lee <bk@golangcn.org>