AMDGPU/SI: Use new SimplifyDemandedBits helper for multi-use operations

Summary:
We are using this helper for our 24-bit arithmetic combines, so we are now able to eliminate multi-use operations that mask the high-bits of 24-bit inputs (e.g. and x, 0xffffff)

Reviewers: arsenm, nhaehnle

Subscribers: tony-tye, arsenm, kzhuravl, wdng, nhaehnle, llvm-commits, yaxunl

Differential Revision: https://reviews.llvm.org/D24672

llvm-svn: 284267