AMDGPU: Make getTgtMemIntrinsic table-driven for resource-based intrinsics

Summary:
Avoids having to list all intrinsics manually.

This is in preparation for the new dimension-aware image intrinsics,
which I'd rather not have to list here by hand.

Change-Id: If7ced04998397ef68c4cb8f7de66b5050fb767e5

Reviewers: arsenm, rampitec, b-sumner

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

Differential Revision: https://reviews.llvm.org/D44937

llvm-svn: 328938