Optimize by demoting "denormalized" Integers (i.e. J# -> S#)

In the course of a recent recent reddit discussion it was highlighted, that integer-gmp doesn't try to demote J# result-values to the more efficient S# even though they would fit into a machine word.

The attached proof-of-concept patch introduces a "smart" J# constructor which constructs a S# value instead (if possible):

-- | Demote 'J#' to 'S#' if possible. See also 'smartJ#'.
toSmall :: Integer -> Integer
toSmall i@(S# _)  = i
toSmall (J# 0# _) = S# 0#
toSmall (J# 1# mb#)  | isTrue# (v ># 0#) = S# v
    where
      v = indexIntArray# mb# 0#
toSmall (J# -1# mb#) | isTrue# (v <# 0#) = S# v
    where
      v = negateInt# (indexIntArray# mb# 0#)
toSmall i         = i

-- | Smart 'J#' constructor which tries to construct 'S#' if possible
smartJ# :: Int# -> ByteArray# -> Integer
smartJ# s# mb# = toSmall (J# s# mb#)

The patch replaces a couple of J#-invocations which are likely to produce a S#-fitting Integer. A nofib comparison for vanilla GHC HEAD vs. patched GHC HEAD is attached for further discussion.