[dev.regabi] cmd/compile: separate misc for gc split

Misc cleanup for splitting package gc: API tweaks
and boundary adjustments.

The change in ir.NewBlockStmt makes it a drop-in
replacement for liststmt.

Change-Id: I9455fe8ccae7d71fe8ccf390ac96672389bf4f3d
Reviewed-on: https://go-review.googlesource.com/c/go/+/279305


Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>