Throw away safety for RSH

This is a complete mess: Flo's right-shift doesn't even remotely match
that of LLVM.  To "fix" the problem I'm just forcing the shift count;
to match width with the data to shift.  This throws away a whole lot
of the safety that LLVM provides, but I'm hoping it won't be a problem
because:

* It shouldn't matter that much: if the count is very large then
  everything will end up 0 anyway.

* libflo's width inference should end up with us being safe here