[InstCombine] fold full-shift of sdiv to icmp+extend

This is a disguised sign-bit test with offset:
(X / +DivC) >> (Width - 1) --> ext (X <= -DivC)
(X / -DivC) >> (Width - 1) --> ext (X >= +DivC)

https://alive2.llvm.org/ce/z/cO8JO4

We don't match/test poison in the sdiv constant because
that would be immediate undefined behavior.