[SLPVectorizer] Fix dependency list

Summary:
DemandedBits was added to the requirements of SLPVectorizer in rL261212
(and various earlier version of it), but the appropriate initialization
statement was accidentally forgotten.

Ref [[ https://github.com/JuliaLang/julia/issues/14998 | JuliaLang/julia#14998 ]].

Patch by Yichao Yu.
Reviewers: mssimpso
Differential Revision: http://reviews.llvm.org/D18152

llvm-svn: 263476