[InstCombine] SimplifyDemandedUseBits - remove lshr node if we only demand known sign bit

This is a lshr equivalent to D122340 - if we don't demand any of the additional sign bits introduced by the ashr, the lshr can be treated as an ashr and we can remove the shift entirely if we only demand already known sign bits.

Another step towards PR21929

https://alive2.llvm.org/ce/z/6f3kjq

Differential Revision: https://reviews.llvm.org/D123118