AMDGPU: Future-proof {raw,struct}.buffer.atomic intrinsics

Summary:
The ISA is really supposed to support 64-bit atomics as well,
so the data type should be an overload.

Mesa doesn't use these atomics yet, in fact I noticed this
issue while trying to use the atomics from Mesa.

Change-Id: I77f58317a085a0d3eb933cc7e99308c48a19f83e

Reviewers: tpr

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

Differential Revision: https://reviews.llvm.org/D52291

llvm-svn: 343978