First, renove unnecessary "// cond:" lines from the generated files.
This shaves off about ~7k lines.

Second, join "if cond { break }" statements via "||", which allows us to
deduplicate a large number of them. This shaves off another ~25k lines.

This change is not for readability or simplicity; but rather, to avoid
unnecessary verbosity that makes the generated files larger. All in all,
git reports that the generated files overall weigh ~200KiB less, or
about 2.7% less.

While at it, add a -trace flag to rulegen.

Updates #33644.

Change-Id: I3fac0290a6066070cc62400bf970a4ae0929470a
Reviewed-on: https://go-review.googlesource.com/c/go/+/196498
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>