[AMDGPU][mc] Add support for buffer_load_dwordx3, buffer_store_dwordx3.

Partially fixes Bug 28232.
Lit tests added.

Differential Revision: https://reviews.llvm.org/D25367

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@283567 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed