cmd/compile: mention that -m can be increased or given multiple times
-m can be increased or it can be given up to 4 times to increase the verbosity of the printed optimization decisions: https://github.com/golang/go/search?q=LowerM
Change-Id: I0cc304385be052664fad455ff075846a3a63f298
GitHub-Last-Rev:
140f08529024dd17c5ca1cbad52dd1d17c6126c0
GitHub-Pull-Request: golang/go#44857
Reviewed-on: https://go-review.googlesource.com/c/go/+/299709
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>