[dev.regabi] cmd/compile: cleanup preparing for concrete types, 2

[dev.regabi] cmd/compile: cleanup preparing for concrete types, 2

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

Passes buildall w/ toolstash -cmp.

Replay of CL 275885, lost to the bad-merge history rewrite.

Change-Id: I0da89502a0bd636b8766f01b6f843c7821b3e9ab
Reviewed-on: https://go-review.googlesource.com/c/go/+/277955
Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>