cmd/compile: teach CSE that new objects are bespoke

cmd/compile: teach CSE that new objects are bespoke

runtime.newobject never returns the same thing twice,
so the resulting value will never be a common subexpression.

This helps when compiling large static data structures
that include pointers, such as maps and slices.
No clear performance impact on other code. (See below.)

For the code in issue #15112:

Before:
  real	1m14.238s
  user	1m18.985s
  sys	0m0.787s

After:
  real	0m47.172s
  user	0m52.248s
  sys	0m0.767s

For the code in issue #15235, size 10k:

Before:
  real	0m44.916s
  user	0m46.577s
  sys	0m0.304s

After:
  real	0m7.703s
  user	0m9.041s
  sys	0m0.316s

Still more work to be done, particularly for #15112.

Updates #15112
Updates #15235

name       old time/op      new time/op      delta
Template        330ms ±11%       333ms ±13%    ~           (p=0.749 n=20+19)
Unicode         148ms ± 6%       152ms ± 8%    ~           (p=0.072 n=18+20)
GoTypes         1.01s ± 7%       1.01s ± 3%    ~           (p=0.583 n=20+20)
Compiler        5.04s ± 2%       5.06s ± 2%    ~           (p=0.314 n=20+20)

name       old user-ns/op   new user-ns/op   delta
Template   444user-ms ±11%  445user-ms ±10%    ~           (p=0.738 n=20+20)
Unicode    215user-ms ± 5%  218user-ms ± 5%    ~           (p=0.239 n=18+18)
GoTypes    1.45user-s ± 3%  1.45user-s ± 4%    ~           (p=0.620 n=20+20)
Compiler   7.23user-s ± 2%  7.22user-s ± 2%    ~           (p=0.901 n=20+19)

name       old alloc/op     new alloc/op     delta
Template       55.0MB ± 0%      55.0MB ± 0%    ~           (p=0.547 n=20+20)
Unicode        37.6MB ± 0%      37.6MB ± 0%    ~           (p=0.301 n=20+20)
GoTypes         177MB ± 0%       177MB ± 0%    ~           (p=0.065 n=20+19)
Compiler        798MB ± 0%       797MB ± 0%  -0.05%        (p=0.000 n=19+20)

name       old allocs/op    new allocs/op    delta
Template         492k ± 0%        493k ± 0%  +0.03%        (p=0.030 n=20+20)
Unicode          377k ± 0%        377k ± 0%    ~           (p=0.423 n=20+19)
GoTypes         1.40M ± 0%       1.40M ± 0%    ~           (p=0.102 n=20+20)
Compiler        5.53M ± 0%       5.53M ± 0%    ~           (p=0.094 n=17+18)

name       old text-bytes   new text-bytes   delta
HelloSize        561k ± 0%        561k ± 0%    ~     (all samples are equal)
CmdGoSize       6.13M ± 0%       6.13M ± 0%    ~     (all samples are equal)

name       old data-bytes   new data-bytes   delta
HelloSize        128k ± 0%        128k ± 0%    ~     (all samples are equal)
CmdGoSize        306k ± 0%        306k ± 0%    ~     (all samples are equal)

name       old exe-bytes    new exe-bytes    delta
HelloSize        905k ± 0%        905k ± 0%    ~     (all samples are equal)
CmdGoSize       9.64M ± 0%       9.64M ± 0%    ~     (all samples are equal)

Change-Id: Id774e2901d7701a3ec7a1c1d1cf1d9327a4107fc
Reviewed-on: https://go-review.googlesource.com/21937
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Todd Neal <todd@tneal.org>