[dev.regabi] cmd/compile: separate CommStmt from CaseStmt

Like go/ast and cmd/compile/internal/syntax before it, package ir now
has separate concrete representations for switch-case clauses and
select-communication clauses.

Passes toolstash -cmp.

Change-Id: I32667cbae251fe7881be0f434388478433b2414f
Reviewed-on: https://go-review.googlesource.com/c/go/+/280443


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>