Poison values [WAS: Re: rotate]

I’m noticing that since I originally submitted this patch, a new keyword has been added… isExact. Particularly in llvm/include/llvm/IRBuilder.h, on the lshr and ashr operators.

I do not suppose that a circular shift can produce a poison value, but this is a little out of my domain. Would someone more familiar with poison values give me a bit of insight to them.