Add tanh lowering from Standard dialect to NVVM and ROCDL.

Summary:
The tanh lowering from Standard dialect to NVVM and ROCDL was not working.
The conversion pattern are inserted in the lowering files.
The test cases for the lowerings were added in the test files.

Reviewers: nicolasvasilache, ftynse, herhut

Reviewed By: ftynse, herhut

Subscribers: merge_guards_bot, ftynse, jholewinski, mehdi_amini, rriddle, jpienaar, burmako, shauheen, antiagainst, csigg, arpith-jacob, mgester, lucyrfox, herhut, liufengdb, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73471