AMDGPU/SI: Improve SILoadStoreOptimizer and run it before the scheduler

Summary:
The SILoadStoreOptimizer can now look ahead more then one instruction when
looking for instructions to merge, which greatly improves the number of
loads/stores that we are able to merge.

Moving the pass before scheduling avoids increasing register pressure after
the scheduler, so that the scheduler's register pressure estimates will be
more accurate.  It also gives more consistent results, since it is no longer
affected by minor scheduling changes.

Reviewers: arsenm

Subscribers: arsenm, kzhuravl, llvm-commits

Differential Revision: https://reviews.llvm.org/D23814

llvm-svn: 279991