Handle exp2 with proper vectorization and lowering to SVML calls

Summary:
Add mapping from exp2 math functions
to corresponding SVML calls.

This is a follow up and extension for llvm diff
https://reviews.llvm.org/D19544

Test Plan:
- update test case and run ninja check.
- run tests locally

Reviewers: wenlei, hoyFB, mmasten, mzolotukhin, spatel

Reviewed By: spatel

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77114