[AMDGPU] Internalize non-kernel symbols

Since we have no call support and late linking we can produce code
only for used symbols. This saves compilation time, size of the final
executable, and size of any intermediate dumps.

Run Internalize pass early in the opt pipeline followed by global
DCE pass. To enable it RT can pass -amdgpu-internalize-symbols option.

Differential Revision: https://reviews.llvm.org/D29214

llvm-svn: 293549