Turn a funnel shift by N in the range `121..128` into a funnel shift in
the opposite direction by `128 - N`. Because there are dedicated
instructions for funnel shifts by values smaller than 8, this emits
fewer instructions.
This additional rule is useful because LLVM appears to canonicalize
`fshr` into `fshl`, meaning that the rules for `fshr` on values less
than 8 would not match on organic input.
