NFC: Refactor library-specific mappings of scalar maths functions to their vector...

NFC: Refactor library-specific mappings of scalar maths functions to their vector counterparts

This patch factors out mappings of scalar maths functions to their vector
counterparts from TargetLibraryInfo.cpp to a separate VecFuncs.def file. Such
mappings are currently available for Accelerate framework, and SVML library.

This is in support of the follow-up: https://reviews.llvm.org/D59881

Patch by pjeeva01

Differential revision: https://reviews.llvm.org/D60211

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@358001 91177308-0d34-0410-b5e6-96231b3b80d8