If all the shifts amount are already poison-producing,

then we can add more poison-producing flags ontop:

https://rise4fun.com/Alive/Ocwi

Otherwise, we should only consider the possible range of shift amts that don't result in poison.

For unsigned range not not overflow, we must not shift out any set bits,

and the actual limit for `x` can be computed by backtransforming

the maximal value we could ever get out of the `shl` - `-1` through

`lshr`. If the `x` is any larger than that then it will overflow.

Likewise for signed range, but just in signed domain..

This is based on the general idea outlined by @nikic in https://reviews.llvm.org/D68672#1714990