[dev.regabi] cmd/compile: change CaseStmt.Vars to Var

There's only ever one variable implicitly declared by a CaseStmt. It's
only a slice because we previous used Rlist for this.

Passes toolstash -cmp.

Change-Id: Idf747f3ec6dfbbe4e94d60546ba04a81754df3fe
Reviewed-on: https://go-review.googlesource.com/c/go/+/280012


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>