runtime: simplify timers.siftDown

runtime: simplify timers.siftDown

No effect on benchmarks, but the code is clearer.

goos: linux
goarch: amd64
pkg: time
cpu: AMD Ryzen 9 7950X 16-Core Processor
                                   │  s7base.txt  │               s7.txt                │
                                   │    sec/op    │    sec/op     vs base               │
AdjustTimers10000-32                 195.9µ ± 13%   198.1µ ±  2%       ~ (p=0.436 n=10)
AdjustTimers10000SingleThread-32     1.573m ± 13%   1.566m ± 10%       ~ (p=0.739 n=10)
AdjustTimers10000NoReset-32          170.6µ ±  1%   170.4µ ±  1%       ~ (p=0.912 n=10)
AdjustTimers10000NoSleep-32          183.9µ ±  2%   181.4µ ±  2%  -1.39% (p=0.045 n=10)
AdjustTimers10000NoResetNoSleep-32   151.3µ ±  1%   150.0µ ±  1%  -0.90% (p=0.007 n=10)

goos: darwin
goarch: arm64
pkg: time
cpu: Apple M3 Pro
                                   │ m3base.txt  │               m3.txt               │
                                   │   sec/op    │   sec/op     vs base               │
AdjustTimers10000-12                 234.2µ ± 1%   234.5µ ± 1%       ~ (p=0.796 n=10)
AdjustTimers10000SingleThread-12     1.191m ± 1%   1.272m ± 1%  +6.81% (p=0.000 n=10)
AdjustTimers10000NoReset-12          239.6µ ± 2%   236.3µ ± 9%       ~ (p=0.971 n=10)
AdjustTimers10000NoSleep-12          223.3µ ± 2%   221.4µ ± 3%       ~ (p=0.579 n=10)
AdjustTimers10000NoResetNoSleep-12   209.2µ ± 2%   209.0µ ± 4%       ~ (p=0.796 n=10)

Change-Id: Id48aa893235d652814b7fa4605037f09b0b4d73b
Reviewed-on: https://go-review.googlesource.com/c/go/+/574897
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>