cmd/compile: clean up ctz ops

cmd/compile: clean up ctz ops

Now that we have ops that can return 2 results, have BSF return a result
and flags.  We can then get rid of the redundant comparison and use CMOV
instead of CMOVconst ops.

Get rid of a bunch of the ops we don't use.  Ctz{8,16}, plus all the Clzs,
and CMOVNEs.  I don't think we'll ever use them, and they would be easy
to add back if needed.

Change-Id: I8858a1d017903474ea7e4002fc76a6a86e7bd487
Reviewed-on: https://go-review.googlesource.com/27630
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>