runtime: optimize siftupTimer and siftdownTimer a bit

runtime: optimize siftupTimer and siftdownTimer a bit

Write the moving timer only once, since it is overwritten
by swapped timers on all the iterations except the last one.

Additionally, explicitly pass timers heap into siftupTimer
and siftdownTimer in order to make the code more clear.

Relevant benchmark results on linux/amd64:

Stop                      700µs ± 7%   608µs ± 1%  -13.13%  (p=0.000 n=10+10)
Stop-2                    440µs ± 4%   376µs ± 4%  -14.48%  (p=0.000 n=10+10)
Stop-4                    339µs ± 2%   330µs ± 3%   -2.66%  (p=0.015 n=10+10)
SimultaneousAfterFunc     702µs ± 9%   709µs ± 1%     ~     (p=0.436 n=9+9)
SimultaneousAfterFunc-2   573µs ± 2%   546µs ± 2%   -4.71%  (p=0.000 n=10+10)
SimultaneousAfterFunc-4   387µs ± 1%   368µs ± 1%   -4.89%  (p=0.000 n=8+10)
StartStop                 268µs ± 0%   270µs ± 0%   +0.91%  (p=0.000 n=9+9)
StartStop-2               155µs ± 6%   145µs ± 6%   -6.70%  (p=0.000 n=10+10)
StartStop-4               125µs ± 1%   124µs ± 1%     ~     (p=0.065 n=10+9)

Change-Id: I3685835b5e3e82844e2e5e73ee03a1e22100bf0e
Reviewed-on: https://go-review.googlesource.com/63110
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>