AMDGPU/SI: Canonicalize offset order for merged DS instructions

Summary:
If the scheduler clusters the loads, then the offsets will be sorted,
but it is possible for the scheduler to scheduler loads together
without out explicitly clustering them, which would give us non-sorted
offsets.

Also, we will want to do this if we move the load/store optimizer before
the scheduler.

Reviewers: arsenm

Subscribers: arsenm, llvm-commits, kzhuravl

Differential Revision: https://reviews.llvm.org/D23776

llvm-svn: 279870