More cleanups after CL 20089

- copysub, take a bool rather than an int for the f (force) parameter.
- copysub returns a bool rather than an int.
- prevl, reg is now int16, which reduces type conversion in its callers.
- copy1, reduce the scope of t and p variables.
- small simplifications in copyau1, copyas, etc.
- {mips64,ppc64}/regzer returns a bool.
- apply CL 20181 to x86/peep.go which was missed in the last CL.
- various comment fixes.

Change-Id: Ib73ffb768c979ce86f1614e5366fd576dea50986
Reviewed-on: https://go-review.googlesource.com/20281Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Dave Cheney <dave@cheney.net>
TryBot-Result: Gobot Gobot <gobot@golang.org>