AMDGPU: Use generic tables instead of SearchableTable

Summary:

Reviewers: arsenm, rampitec

Subscribers: kzhuravl, wdng, yaxunl, dstuttard, tpr, t-tye, llvm-commits

Differential Revision: https://reviews.llvm.org/D48014

Change-Id: Ibb43f90d955275571aff17d0c3ecfb5e5b299641
llvm-svn: 335226