ghc.compiler.GHC.Cmm.Switch.restrictMap could be more efficient

The first split

restrictMap :: (Integer,Integer) -> M.Map Integer b -> M.Map Integer b 
restrictMap (lo,hi) m = mid   where
  (_,   mid_hi) = M.split (lo-1) m
  (mid, _) =      M.split (hi+1) mid_hi

might do useless work when the result map is small. See discussion at https://github.com/haskell/containers/pull/803#issuecomment-1062968905

But .. if it did not show up in profiles before, then perhaps it does not need to be fixed. Anyway I thought I'd mention it here.

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