[AMDGPU] Supported ds_write_b128 generation.

Summary: This is a follow-on patch of https://reviews.llvm.org/D44210

Author: FarhanaAleen

Reviewed By: msearles

Subscribers: llvm-commits, AMDGPU

Differential Revision: https://reviews.llvm.org/D44319

llvm-svn: 327726