[dev.regabi] cmd/compile: remove CommStmt.List

Package syntax's parser already ensures that select communication
clauses only have one statement, so there's no need for ir's CommStmt
to need to represent more than one. Instead, noder can just directly
populate Comm in the first place.

Incidentally, this also revealed a latent issue in the inline-body
exporter: we were exporting List (where the case statement is before
type-checking), rather than Comm (where the case statement would be
after type-checking, when export happens).

Passes toolstash -cmp.

Change-Id: Ib4eb711527bed297c7332c79ed6e6562a1db2cfa
Reviewed-on: https://go-review.googlesource.com/c/go/+/280444


Trust: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>