Draft: Change CaloCellID to denseIndex conversions to be SIMD friendly

Arthur Marius Hennequin requested to merge ahennequ_calocellid into master

@graven @cmarinbe @mveghel

  • Avoid having too many select() (prefer boolean operations)
  • Use power of two widths so we can avoid integer division (which doesn't exist for avx2)
  • Use unsigned overflows to reduce the number of comparisons in validity check
  • Use a substractive check instead of an additive one

For SIMD version of validity check see https://godbolt.org/z/5KeY9Wdhf

Goes with Rec!2662

Edited by Arthur Marius Hennequin

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