GeneralPurposeAllocator.searchBucket: check current bucket before searching the list

[squeek502]

Follow up to #17383. This is a minor optimization that only matters when a small allocation is resized/free'd soon after it is allocated.

The only real difference I was able to observe with this was via a synthetic benchmark that allocates a full bucket and then frees all but one of the slots, over and over in a loop (see "Benchmark code" in #17383):

Debug build:

Benchmark 1 (9 runs): gpa-degen-master.exe
  measurement          mean ± σ            min … max           outliers         delta
  wall_time           575ms ± 5.19ms     569ms …  583ms          0 ( 0%)        0%
  peak_rss           43.8MB ± 1.37KB    43.8MB … 43.8MB          1 (11%)        0%
Benchmark 2 (10 runs): gpa-degen-search-cur.exe
  measurement          mean ± σ            min … max           outliers         delta
  wall_time           532ms ± 5.55ms     520ms …  539ms          0 ( 0%)        ⚡-  7.5% ±  0.9%
  peak_rss           43.8MB ± 65.2KB    43.8MB … 44.0MB          1 (10%)          +  0.0% ±  0.1%

ReleaseFast build:

Benchmark 1 (129 runs): gpa-degen-master-release.exe
  measurement          mean ± σ            min … max           outliers         delta
  wall_time          38.9ms ± 1.12ms    36.7ms … 42.4ms          8 ( 6%)        0%
  peak_rss           23.2MB ± 2.39KB    23.2MB … 23.2MB          0 ( 0%)        0%
Benchmark 2 (151 runs): gpa-degen-search-cur-release.exe
  measurement          mean ± σ            min … max           outliers         delta
  wall_time          33.2ms ±  999us    31.9ms … 36.3ms         20 (13%)        ⚡- 14.7% ±  0.6%
  peak_rss           23.2MB ± 2.26KB    23.2MB … 23.2MB          0 ( 0%)          +  0.0% ±  0.0%

No difference in Debug mode standard library tests:

Benchmark 1 (10 runs): std-tests-master.exe
  measurement          mean ± σ            min … max           outliers         delta
  wall_time          16.1s  ±  224ms    16.0s  … 16.7s           1 (10%)        0%
  peak_rss           42.3MB ± 4.65KB    42.3MB … 42.3MB          1 (10%)        0%
Benchmark 2 (10 runs): std-tests-search-cur.exe
  measurement          mean ± σ            min … max           outliers         delta
  wall_time          16.0s  ± 43.8ms    16.0s  … 16.1s           0 ( 0%)          -  0.4% ±  0.9%
  peak_rss           42.3MB ±  181KB    42.3MB … 42.8MB          1 (10%)          +  0.1% ±  0.3%

No noticeable difference in the degraded arocc case from #17383 (this is on Windows with stack trace collection turned off):

Benchmark 1: testaro-suffix.bat master
  Time (mean ± σ):      6.658 s ±  0.054 s    [User: 5.243 s, System: 1.407 s]
  Range (min … max):    6.606 s …  6.746 s    5 runs

Benchmark 2: testaro.bat
  Time (mean ± σ):      6.609 s ±  0.055 s    [User: 5.124 s, System: 1.451 s]
  Range (min … max):    6.540 s …  6.684 s    5 runs

Summary
  'testaro.bat' ran
    1.01 ± 0.01 times faster than 'testaro-suffix.bat master'