cmd/compile/ssa: generate less garbage in schedule

cmd/compile/ssa: generate less garbage in schedule

Passes toolstash -cmp.

name       old alloc/op    new alloc/op    delta
Template      58.5MB ± 0%     57.8MB ± 0%  -1.15%        (p=0.000 n=10+10)
Unicode       41.3MB ± 0%     41.2MB ± 0%  -0.17%        (p=0.000 n=10+10)
GoTypes        196MB ± 0%      193MB ± 0%  -1.26%        (p=0.000 n=10+10)
Compiler       863MB ± 0%      850MB ± 0%  -1.49%        (p=0.000 n=10+10)

name       old allocs/op   new allocs/op   delta
Template        522k ± 0%       507k ± 0%  -2.99%        (p=0.000 n=10+10)
Unicode         403k ± 0%       401k ± 0%  -0.42%        (p=0.000 n=10+10)
GoTypes        1.58M ± 0%      1.52M ± 0%  -3.61%        (p=0.000 n=10+10)
Compiler       6.47M ± 0%      6.17M ± 0%  -4.62%        (p=0.000 n=10+10)

Change-Id: Ia7a6242e8d226b41966c344d253814dcce6424a8
Reviewed-on: https://go-review.googlesource.com/21141
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>