Changed wrong ROCDL instructions in GPU lowering.

Summary:
In the scope of the lowering phase from GPU to ROCDL, the intructions for the conversion patterns seems to be wrong.
According to https://github.com/ROCm-Developer-Tools/HIP/blob/master/include/hip/hcc_detail/math_fwd.h the instructions need two underscores in the beginning instead of one.

Reviewers: nicolasvasilache, herhut, rriddle

Reviewed By: herhut, rriddle

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73535