cmd/gc: make liveness ~10x faster

1) The arrayindexof lookup function is O(n). Replace with O(1) lookups.

2) The checkptxt function is O(n²) and is purely for debugging.
Only run when the debugging flags are turned on.

3) Iterating over sparse bitmaps can be done faster word by word.
Introduce and use bvnext for that.

Run times before and after, on my 2.5 GHz Core i5 MacBook Pro.

x.go       9.48  0.84  issue 8259

x100.go    0.01  0.01  issue 8354
x1000.go   0.10  0.10
x2000.go   0.62  0.19
x3000.go   1.33  0.34
x4000.go   2.29  0.49
x5000.go   3.89  0.67
x6000.go   5.00  0.90
x7000.go   6.70  1.13
x8000.go   9.44  1.38
x9000.go  11.23  1.87
x10000.go 13.78  2.09

Fixes issue 8259.
Fixes issue 8354.

LGTM=iant, r
R=golang-codereviews, iant, r
CC=golang-codereviews
https://codereview.appspot.com/125720043
codereview: fix submit of merge + include branch prefix in submitted message

hg insists that we not list explicit files for a merge.

for benefit of other tools reading logs, include branch prefix
at start of every commit message.

LGTM=minux
R=minux
CC=golang-codereviews
https://codereview.appspot.com/124780044
codereview: do not hit upload_complete on first patch

Causes server 500 error, so don't do it.

TBR=minux
CC=golang-codereviews
https://codereview.appspot.com/125770043
[dev.power64] all: merge default, for codereview update

LGTM=minux
R=minux
CC=golang-codereviews
https://codereview.appspot.com/124820043
[dev.power64] cmd/dist: recognize power64 and power64le as GOHOSTARCH

LGTM=rsc
R=rsc, iant
CC=golang-codereviews
https://codereview.appspot.com/125760043