mulmayoflo fails for W8 and W16 on X86 NCG

test-primops reports this panic:

ghc-9.4.5: panic! (the 'impossible' happened)
  GHC version 9.4.5:
        shift_amt

Please report this as a GHC bug:  https://www.haskell.org/ghc/reportabug

It seems nobody implemented shift_amt for W8 and W16:

             shift_amt  = case rep of
                           W32 -> 31
                           W64 -> 63
                           _ -> panic "shift_amt"

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