[mlir] Add support for lowering tanh to LLVMIR.

Summary:
Add a pattern for expanding tanh op into exp form.
A `tanh` is expanded into:
   1) 1-exp^{-2x} / 1+exp^{-2x}, if x => 0
   2) exp^{2x}-1 / exp^{2x}+1  , if x < 0.

Differential Revision: https://reviews.llvm.org/D81618