[dev.regabi] cmd/compile: ir.BranchStmt, add ir.EmptyStmt, ir.LabelStmt

[dev.regabi] cmd/compile: ir.BranchStmt, add ir.EmptyStmt, ir.LabelStmt

These are the first three specific implementations of Node.
They are both a bit of a warmup and also working toward
removing references to Name from Node types other than
the proper named things - ONAME, ONONAME, OTYPE, OLITERAL.
(In this case, BranchStmt and LabelStmt.)

Passes buildall w/ toolstash -cmp.

Change-Id: Ide816b162025ee4c858dd061d7c29ed633fb7baf
Reviewed-on: https://go-review.googlesource.com/c/go/+/274091
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>