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Opened by Moritz Angermann@angermanDeveloper

Fold away or reduce `and# (extendWordN# x) mask`

We may end up with expressions like:

(GHC.Prim.uncheckedShiftL#
 (GHC.Prim.and#
  (GHC.Prim.extendWord8 x)
  18446744073709551487##)
  ...)

We know x is of Word8, so we can truncate the mask (0xFFFFFF7F) to 0x7F. And subsequently if the mask was 0xFF we could just drop the and# altogether, independent if the upper parts of the mask are not set.