[AMDGPU] Increased vector length for global/constant loads.

Summary: GCN ISA supports instructions that can read 16 consecutive dwords from memory through the scalar data cache;
         loadstoreVectorizer should take advantage of the wider vector length and pack 16/8 elements of dwords/quadwords.

Author: FarhanaAleen

Reviewed By: rampitec

Subscribers: llvm-commits, AMDGPU

Differential Revision: https://reviews.llvm.org/D44179

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@326910 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed