Determine SIMD level in the planner

[Shnatsel]

Detect the SIMD level once and store it for later. Up to 10% better perf on x86, mostly on small sizes.

No change on ARM. I feared it might regress something by making it more dynamic, but no.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.78%. Comparing base (7aaba0f) to head (04f73a9).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #69   +/-   ##
=======================================
  Coverage   99.78%   99.78%           
=======================================
  Files          13       13           
  Lines        2844     2846    +2     
=======================================
+ Hits         2838     2840    +2     
  Misses          6        6           

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