scheduling dependencies. Add assertion checks to help catch
this.
It appears the Mips target defaults to list-td, and it has a
regression test that uses a physreg dependence. Such code was
liable to be miscompiled, and now evokes an assertion failure.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@62177 91177308-0d34-0410-b5e6-96231b3b80d8
