Fast tanh for the LLVM backend. (#39528)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/39528
Test Plan: Imported from OSS
Differential Revision: D21927791
Pulled By: zheng-xq
fbshipit-source-id: a3f7d79bf0d3a399000ffd7ff4d0502ba365f1dc
