Reapply "[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.

Reapplying with a smaller number of inline elements in the
SmallSetVector, to avoid running into the SmallDenseMap issue
described in D56455.

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

llvm-svn: 350997