Add a catch-all WriteSystem SchedWrite type.

This is used for all the expensive system instructions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177598 91177308-0d34-0410-b5e6-96231b3b80d8
Model prefetches and barriers as loads.

It's not yet clear if these instructions need a more careful model.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177599 91177308-0d34-0410-b5e6-96231b3b80d8