[VFDatabase] Scalar functions are vector functions with VF =1

Summary:
Return scalar function when VF==1. The new trivial mapping scalar --> scalar when VF==1 to prevent false positive for "isVectorizable" query.

Author: masoud.ataei (Masoud Ataei)

Reviewers: Whitney (Whitney Tsang), fhahn (Florian Hahn), pjeeva01 (Jeeva P.), fpetrogalli (Francesco Petrogalli), rengolin (Renato Golin)

Reviewed By: fpetrogalli (Francesco Petrogalli)

Subscribers: hiraditya (Aditya Kumar), llvm-commits, LLVM

Tag: LLVM

Differential Revision: https://reviews.llvm.org/D78054