cmd/compile: allocate Nodes together with Name/Param/Func

cmd/compile: allocate Nodes together with Name/Param/Func

After allocating a Node that needs a Name, Param, and/or Func field,
we never clear that field, so we can reduce GC overhead slightly by
allocating them together with the owner Node.

name       old time/op     new time/op     delta
Template       325ms ± 7%      325ms ± 7%    ~           (p=0.910 n=29+30)
Unicode        177ms ±12%      173ms ±11%    ~           (p=0.110 n=29+30)
GoTypes        1.06s ± 7%      1.05s ± 5%  -1.22%        (p=0.027 n=30+30)
Compiler       4.48s ± 3%      4.47s ± 3%    ~           (p=0.423 n=30+30)

name       old user-ns/op  new user-ns/op  delta
Template        476M ±22%       467M ±14%    ~           (p=0.310 n=29+30)
Unicode         298M ±22%       294M ±25%    ~           (p=0.335 n=30+30)
GoTypes        1.54G ± 9%      1.48G ± 9%  -4.06%        (p=0.000 n=30+30)
Compiler       6.26G ± 6%      6.14G ± 6%  -1.90%        (p=0.004 n=30+30)

name       old alloc/op    new alloc/op    delta
Template      40.9MB ± 0%     41.1MB ± 0%  +0.53%        (p=0.000 n=30+30)
Unicode       30.9MB ± 0%     31.0MB ± 0%  +0.16%        (p=0.000 n=30+30)
GoTypes        122MB ± 0%      123MB ± 0%  +0.37%        (p=0.000 n=30+30)
Compiler       477MB ± 0%      479MB ± 0%  +0.37%        (p=0.000 n=30+29)

name       old allocs/op   new allocs/op   delta
Template        400k ± 1%       376k ± 1%  -5.96%        (p=0.000 n=30+30)
Unicode         330k ± 1%       325k ± 1%  -1.48%        (p=0.000 n=30+30)
GoTypes        1.22M ± 0%      1.16M ± 0%  -4.38%        (p=0.000 n=30+30)
Compiler       4.35M ± 0%      4.13M ± 0%  -5.08%        (p=0.000 n=30+29)

Change-Id: I9bdc7d9223bb32f785df71810564e82d9a76d109
Reviewed-on: https://go-review.googlesource.com/36022
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>