cmd/compile/internal/syntax: check fallthrough in CheckBranches mode

cmd/compile/internal/syntax: check fallthrough in CheckBranches mode

The parser CheckBranches mode checked correct use of break, continue,
and labels, but not of fallthrough statements.

This CL adds checking of fallthrough statements as well.

For #51456.

Change-Id: I5000388011973724f80c59a6aaf015e3bb70faea
Reviewed-on: https://go-review.googlesource.com/c/go/+/414134
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>