[AMDGPU] Supported ds_read_b128 generation; Widened vector length for local address-space.

Summary: Starting from GCN 2nd generation, ISA supports ds_read_b128 on top of ds_read_b64.
         This patch supports ds_read_b128 instruction pattern and generation of this instruction.
         In the vectorizer, this patch also widen the vector length so that vectorizer generates
         128 bit loads for local address-space which gets translated to ds_read_b128.
         Since the performance benefit is not clear; compiler generates ds_read_b128 under -amdgpu-ds128.

Author: FarhanaAleen

Reviewed By: rampitec, arsenm

Subscribers: llvm-commits, AMDGPU

Differential Revision: https://reviews.llvm.org/D44210

llvm-svn: 327153