Revert "Allow MachineCSE to coalesce trivial subregister copies the same way that it coalesces normal copies."

This reverts commit r197414.

It broke the ppc64 bootstrap. I will post a testcase in a sec.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197424 91177308-0d34-0410-b5e6-96231b3b80d8