Int64/Word64 constant folding (#19024)

Closed Sylvain Henry requested to merge hsyl20/ghc:hsyl20/word64-constantfold into master
  • Replace FFI calls with primops for Word64/Int64
  • Add constant folding rules

This is needed to avoid regressions in !5862 (closed) (removing fromIntegral rules) and also in !3658 (closed) (making Int64#/word64# available on every architecture).

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