cmd/compile: add OpOffPtr [c] SP to constant cache

cmd/compile: add OpOffPtr [c] SP to constant cache

They accounted for almost 30% of all CSE'd values.

By never creating the duplicates in the first place,
we reduce the high water mark of Value IDs,
which in turn makes all SSA phases cheaper,
particularly regalloc.

name       old time/op     new time/op     delta
Template       200ms ± 3%      198ms ± 4%  -0.87%  (p=0.016 n=50+49)
Unicode       86.9ms ± 2%     85.5ms ± 3%  -1.56%  (p=0.000 n=49+50)
GoTypes        553ms ± 4%      551ms ± 4%    ~     (p=0.183 n=50+49)
SSA            3.97s ± 3%      3.93s ± 2%  -1.06%  (p=0.000 n=48+48)
Flate          124ms ± 4%      124ms ± 3%    ~     (p=0.545 n=48+50)
GoParser       146ms ± 4%      146ms ± 4%    ~     (p=0.810 n=49+49)
Reflect        357ms ± 3%      355ms ± 3%  -0.59%  (p=0.049 n=50+48)
Tar            106ms ± 4%      107ms ± 5%    ~     (p=0.454 n=49+50)
XML            203ms ± 4%      203ms ± 4%    ~     (p=0.726 n=48+50)

name       old user-ns/op  new user-ns/op  delta
Template        237M ± 3%       235M ± 4%    ~     (p=0.208 n=47+48)
Unicode         111M ± 4%       108M ± 9%  -2.50%  (p=0.000 n=47+50)
GoTypes         736M ± 5%       729M ± 4%  -0.95%  (p=0.017 n=50+46)
SSA            5.73G ± 4%      5.74G ± 4%    ~     (p=0.765 n=50+50)
Flate           150M ± 5%       148M ± 6%  -0.89%  (p=0.045 n=48+47)
GoParser        180M ± 5%       178M ± 7%  -1.34%  (p=0.012 n=50+50)
Reflect         450M ± 4%       444M ± 4%  -1.40%  (p=0.000 n=50+49)
Tar             124M ± 7%       123M ± 7%    ~     (p=0.092 n=50+50)
XML             248M ± 6%       245M ± 5%    ~     (p=0.057 n=50+50)

name       old alloc/op    new alloc/op    delta
Template      39.4MB ± 0%     39.3MB ± 0%  -0.37%  (p=0.000 n=50+50)
Unicode       30.9MB ± 0%     30.9MB ± 0%  -0.27%  (p=0.000 n=48+50)
GoTypes        114MB ± 0%      113MB ± 0%  -1.03%  (p=0.000 n=50+49)
SSA            882MB ± 0%      865MB ± 0%  -1.95%  (p=0.000 n=49+49)
Flate         25.8MB ± 0%     25.7MB ± 0%  -0.21%  (p=0.000 n=50+50)
GoParser      31.7MB ± 0%     31.6MB ± 0%  -0.33%  (p=0.000 n=50+50)
Reflect       79.7MB ± 0%     79.3MB ± 0%  -0.49%  (p=0.000 n=44+49)
Tar           27.2MB ± 0%     27.1MB ± 0%  -0.31%  (p=0.000 n=50+50)
XML           42.7MB ± 0%     42.3MB ± 0%  -1.05%  (p=0.000 n=48+49)

name       old allocs/op   new allocs/op   delta
Template        379k ± 1%       380k ± 1%  +0.26%  (p=0.000 n=50+50)
Unicode         324k ± 1%       324k ± 1%    ~     (p=0.964 n=49+50)
GoTypes        1.14M ± 0%      1.15M ± 0%  +0.14%  (p=0.000 n=50+49)
SSA            7.89M ± 0%      7.89M ± 0%  -0.05%  (p=0.000 n=49+49)
Flate           240k ± 1%       241k ± 1%  +0.27%  (p=0.001 n=50+50)
GoParser        310k ± 1%       311k ± 1%  +0.48%  (p=0.000 n=50+49)
Reflect        1.00M ± 0%      1.00M ± 0%  +0.17%  (p=0.000 n=48+50)
Tar             254k ± 1%       255k ± 1%  +0.23%  (p=0.005 n=50+50)
XML             395k ± 1%       395k ± 1%  +0.19%  (p=0.002 n=49+47)

Change-Id: Iaa8f5f37e23bd81983409f7359f9dcd4dfe2961f
Reviewed-on: https://go-review.googlesource.com/38003
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>