[AMDGPU] Add custom lowering for llvm.log{,10}.{f16,f32} intrinsics

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