Don't use int2Word# -1# in integer-gmp; the compiler misunderstands.
Stop GHC from accidentally generating 64 bit Word or Int literals.

The real fix would be to kick GHC somewhere; this stopgap measure just ensures
that GHC can't inline word2Int# and int2Word# and so can't turn
int2Word# -1# into 0xffffffffffffffff##. We do the inlining in the
code generator instead.
Only show const overflow warnings with --verbose.

They're all from deriving Typeable anyway now; users will just have to live
with 64 bit type fingerprints for now.