cmd/compile: use List instead of OKEY for OSLICE*

cmd/compile: use List instead of OKEY for OSLICE*

Performance changes are negligible, but that's expected.
This is a part of a general effort to eliminate OKEY nodes.

Passes toolstash -cmp.

Updates #15350

name       old alloc/op     new alloc/op     delta
Template       40.6MB ± 0%      40.6MB ± 0%  -0.04%         (p=0.000 n=9+10)
Unicode        33.4MB ± 0%      33.4MB ± 0%    ~           (p=0.853 n=10+10)
GoTypes         120MB ± 0%       120MB ± 0%  -0.03%         (p=0.000 n=9+10)
Compiler        470MB ± 0%       469MB ± 0%  -0.06%        (p=0.000 n=10+10)

name       old allocs/op    new allocs/op    delta
Template         404k ± 0%        404k ± 0%    ~           (p=0.165 n=10+10)
Unicode          350k ± 0%        350k ± 0%    ~            (p=0.211 n=9+10)
GoTypes         1.21M ± 0%       1.21M ± 0%    ~           (p=0.315 n=10+10)
Compiler        4.35M ± 0%       4.35M ± 0%  -0.03%        (p=0.001 n=10+10)

Change-Id: I17d547bf9568b1ee2514a7ffab930424617f995e
Reviewed-on: https://go-review.googlesource.com/32213
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>