cmd/compile: replace opnames with stringer

cmd/compile: replace opnames with stringer

Now possible, since stringer just got the -trimprefix flag added.

While at it, simplify a few Op stringifications since we can now use %v,
and no longer have to worry about o<len(opnames).

Passes toolstash -cmp on std cmd.

Fixes #15462.

Change-Id: Icdcde0b0a5eb165d18488918175024da274f782b
Reviewed-on: https://go-review.googlesource.com/76790
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Dave Cheney <dave@cheney.net>