Auto merge of #86823 - the8472:opt-chunk-tra, r=kennytm
Optimize unchecked indexing into chunks and chunks_mut
Fixes #53340
```
# BEFORE
$ rustc +nightly -Copt-level=3 -Ccodegen-units=1 -Clto=fat chunks.rs
$ perf stat ./chunks
Performance counter stats for './chunks':
3,177.03 msec task-clock # 1.000 CPUs utilized
4 context-switches # 0.001 K/sec
0 cpu-migrations # 0.000 K/sec
984,006 page-faults # 0.310 M/sec
13,092,199,322 cycles # 4.121 GHz (83.29%)
384,543,475 stalled-cycles-frontend # 2.94% frontend cycles idle (83.35%)
7,414,280,722 stalled-cycles-backend # 56.63% backend cycles idle (83.38%)
50,493,980,662 instructions # 3.86 insn per cycle
# 0.15 stalled cycles per insn (83.29%)
6,625,375,297 branches # 2085.396 M/sec (83.38%)
3,087,652 branch-misses # 0.05% of all branches (83.31%)
3.178079469 seconds time elapsed
2.327156000 seconds user
0.762041000 seconds sys
# AFTER
$ ./build/x86_64-unknown-linux-gnu/stage1/bin/rustc -Copt-level=3 -Ccodegen-units=1 -Clto=fat chunks.rs
$ perf stat ./chunks
Performance counter stats for './chunks':
2,705.76 msec task-clock # 1.000 CPUs utilized
4 context-switches # 0.001 K/sec
0 cpu-migrations # 0.000 K/sec
984,005 page-faults # 0.364 M/sec
11,156,763,039 cycles # 4.123 GHz (83.26%)
342,198,882 stalled-cycles-frontend # 3.07% frontend cycles idle (83.37%)
6,486,263,637 stalled-cycles-backend # 58.14% backend cycles idle (83.37%)
40,553,476,617 instructions # 3.63 insn per cycle
# 0.16 stalled cycles per insn (83.37%)
6,668,429,113 branches # 2464.532 M/sec (83.37%)
3,099,636 branch-misses # 0.05% of all branches (83.26%)
2.706725288 seconds time elapsed
1.782083000 seconds user
0.848424000 seconds sys
```
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