[DemandedBits] Use SetVector for Worklist

DemandedBits currently uses a simple vector for the worklist, which
means that instructions may be inserted multiple times into it.
Especially in combination with the deep lattice, this may cause
instructions too be recomputed very often. To avoid this, switch
to a SetVector.

Differential Revision: https://reviews.llvm.org/D56362

llvm-svn: 350547