AMDGPU backend errors with "unsupported call to function" upon
encountering a call to llvm.log{,10}.{f16,f32} intrinsics. This patch
adds custom lowering to avoid that error on both R600 and SI.
Reviewers: arsenm, jvesely
Subscribers: tstellar
Differential Revision: https://reviews.llvm.org/D29942
llvm-svn: 319025
Vedran Miletic