[VectorUtils] Move ToVectorTy to VectorUtils.h (NFC).

ToVectorTy is defined and used in multiple places. Hoist it to
VectorUtils.h to avoid duplication and improve re-usability.

Reviewers: rengolin, hsaito, Ayal, gilr, fpetrogalli

Reviewed By: fpetrogalli

Differential Revision: https://reviews.llvm.org/D74959