cmd/compile: stop adding implicit OKEY nodes

cmd/compile: stop adding implicit OKEY nodes

Keys are uncommon in array and slice literals, and normalizing
OARRAYLIT and OSLICELIT nodes to always use OKEY ends up not reducing
complexity much. Instead, only create OKEY nodes to represent explicit
keys, and recalculate implicit keys when/where necessary.

Fixes #15350.

name       old time/op     new time/op     delta
Template       299ms ± 9%      299ms ±12%    ~           (p=0.694 n=28+30)
Unicode        165ms ± 7%      162ms ± 9%    ~           (p=0.084 n=27+27)
GoTypes        950ms ± 9%      963ms ± 5%    ~           (p=0.301 n=30+29)
Compiler       4.23s ± 7%      4.17s ± 7%    ~           (p=0.057 n=29+27)

name       old user-ns/op  new user-ns/op  delta
Template        389M ±15%       400M ±12%    ~           (p=0.202 n=30+29)
Unicode         246M ±21%       232M ±22%  -5.76%        (p=0.006 n=28+29)
GoTypes        1.34G ± 8%      1.34G ± 7%    ~           (p=0.775 n=28+30)
Compiler       5.91G ± 6%      5.87G ± 7%    ~           (p=0.298 n=28+29)

name       old alloc/op    new alloc/op    delta
Template      41.2MB ± 0%     41.2MB ± 0%    ~           (p=0.085 n=30+30)
Unicode       34.0MB ± 0%     31.5MB ± 0%  -7.28%        (p=0.000 n=30+29)
GoTypes        121MB ± 0%      121MB ± 0%    ~           (p=0.657 n=30+30)
Compiler       511MB ± 0%      511MB ± 0%  -0.01%        (p=0.001 n=29+29)

name       old allocs/op   new allocs/op   delta
Template        390k ± 0%       390k ± 0%    ~           (p=0.225 n=30+29)
Unicode         318k ± 0%       293k ± 0%  -8.03%        (p=0.000 n=30+29)
GoTypes        1.16M ± 0%      1.16M ± 0%    ~           (p=0.745 n=30+30)
Compiler       4.35M ± 0%      4.35M ± 0%    ~           (p=0.105 n=30+30)

Change-Id: I6310739a0bfdb54f1ab8a460b2c03615ad1ff5bc
Reviewed-on: https://go-review.googlesource.com/32221
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>