cmd/compile/internal/gc: change bvfoo functions into bvec methods

cmd/compile/internal/gc: change bvfoo functions into bvec methods

plive.go (except for printeffects and livenessprintblock) and
reflect.go changes were prepared mechanically with gofmt -r.

Passes toolstash.

name       old alloc/op    new alloc/op    delta
Template      44.3MB ± 0%     44.3MB ± 0%    ~           (p=0.367 n=30+30)
Unicode       37.4MB ± 0%     37.4MB ± 0%    ~           (p=0.665 n=30+30)
GoTypes        125MB ± 0%      125MB ± 0%    ~           (p=0.067 n=30+30)
Compiler       515MB ± 0%      515MB ± 0%    ~           (p=0.542 n=30+28)

name       old allocs/op   new allocs/op   delta
Template        434k ± 0%       434k ± 0%    ~           (p=0.076 n=30+29)
Unicode         367k ± 0%       367k ± 0%    ~           (p=0.716 n=29+30)
GoTypes        1.24M ± 0%      1.24M ± 0%    ~           (p=0.428 n=29+29)
Compiler       4.47M ± 0%      4.47M ± 0%    ~           (p=0.225 n=28+30)

Change-Id: Ibaf0668567b3f69fba06aa03b7997c8fb152113a
Reviewed-on: https://go-review.googlesource.com/30356
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>