[InstCombine] fold exact sdiv to ashr

sdiv exact X, (1<<ShAmt) --> ashr exact X, ShAmt (if shl is non-negative)

https://alive2.llvm.org/ce/z/kB6VF7

It would probably be better to use ValueTracking to replace this
and the existing transform above it, but the analysis does not
account for the no-wrap properly, and it's not immediately clear
to me how to fix it.