[ConstantFolding] Use hasVectorInstrinsicScalarOpd. NFC

Summary:
Use the hasVectorInstrinsicScalarOpd helper function
in ConstantFoldVectorCall.

Reviewers: rengolin, RKSimon, dblaikie

Reviewed By: rengolin, RKSimon

Subscribers: tschuett, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63705

llvm-svn: 364178