[dev.regabi] cmd/compile: cleanup preparing for concrete types

[dev.regabi] cmd/compile: cleanup preparing for concrete types

Avoid using the same variable for two different concrete
Node types in walk. This will smooth the introduction of
specific constructors, replacing ir.Nod and friends.

Passes buildall w/ toolstash -cmp.

Replay of CL 275884, lost to the bad-merge history rewrite.

Change-Id: I05628e20a19c9559ed7478526ef6cb2613f735e5
Reviewed-on: https://go-review.googlesource.com/c/go/+/277954
Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>