[dev.regabi] cmd/compile: use *ir.Name for Decl.X

Passes toolstash -cmp.

Change-Id: I505577d067eda3512f6d78618fc0eff061a71e3c
Reviewed-on: https://go-review.googlesource.com/c/go/+/280732


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>