R600/SI: Add load / store machine optimizer pass.

Currently this only functions to match simple cases
where ds_read2_* / ds_write2_* instructions can be used.

In the future it might match some of the other weird
load patterns, such as direct to LDS loads.

Currently enabled only with a subtarget feature to enable
easier testing.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219533 91177308-0d34-0410-b5e6-96231b3b80d8