Optimise plane::as_region

[shssoichiro]

This is part of a series of commits authored by @maj160 to improve performance of rav1e.

This commit results in an overall performance improvement of about 2% at speed 2 10-bit:

Benchmark 1: ~/Downloads/rav1e_master -s 2 --quantizer 64 ~/xiph-media-files/objective-1-fast-10bit/speed_bag_640x360_60f.y4m -y -o /dev/null
  Time (mean ± σ):     43.646 s ±  0.088 s    [User: 43.656 s, System: 0.170 s]
  Range (min … max):   43.471 s … 43.787 s    10 runs

Benchmark 2: ~/Downloads/rav1e_mod -s 2 --quantizer 64 ~/xiph-media-files/objective-1-fast-10bit/speed_bag_640x360_60f.y4m -y -o /dev/null
  Time (mean ± σ):     42.919 s ±  0.082 s    [User: 42.934 s, System: 0.169 s]
  Range (min … max):   42.813 s … 43.039 s    10 runs

[codecov-commenter]

Codecov Report

Base: 86.93% // Head: 86.91% // Decreases project coverage by -0.02% ⚠️

Coverage data is based on head (8cc0b24) compared to base (4fb2a27).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3091      +/-   ##
==========================================
- Coverage   86.93%   86.91%   -0.03%     
==========================================
  Files          83       83              
  Lines       33035    33050      +15     
==========================================
+ Hits        28720    28725       +5     
- Misses       4315     4325      +10     

Impacted Files	Coverage Δ
src/frame/plane.rs	100.00% <100.00%> (ø)
src/tiling/plane_region.rs	88.05% <100.00%> (+0.41%)	⬆️
src/cpu_features/x86.rs	39.58% <0.00%> (-2.09%)	⬇️
src/asm/x86/lrf.rs	92.22% <0.00%> (-1.90%)	⬇️
src/context/mod.rs	91.57% <0.00%> (-1.06%)	⬇️
src/rdo.rs	85.62% <0.00%> (ø)
src/me.rs	95.70% <0.00%> (+0.09%)	⬆️

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