[Scalarizer] Handle scalar arguments in vector GEP

Summary:
Triggered by commit r298620: "[LV] Vectorize GEPs".

If we encounter a vector GEP with scalar arguments, we splat the scalar
into a vector of appropriate size before we scatter the argument.

Reviewers: arsenm, mehdi_amini, bkramer

Reviewed By: arsenm

Subscribers: bjope, mssimpso, wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D31416

llvm-svn: 299186