[AMDGPU] Add support for 64 bit buffer atomic artihmetic instructions

Summary:
This adds support for 64 bit buffer atomic arithmetic instructions but does not include
cmpswap as that depends on a fix to the way the register pairs are handled

Change-Id: Ib207ea65fb69487ccad5066ea647ae8ddfe2ce61

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58918

llvm-svn: 355520