R600/SI: Define a schedule model and enable the generic machine scheduler

R600/SI: Define a schedule model and enable the generic machine scheduler

The schedule model is not complete yet, and could be improved.

This is a partial merge of r227461.  The difference is that it
does not enable the machine scheduler by default.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/branches/release_36@227596 91177308-0d34-0410-b5e6-96231b3b80d8