Disabled subregister copy coalescing during MachineCSE.

This effectively backs out r197465 but leaves some of the general
fixes in place. Not all targets are ready to handle this feature. To
enable it, some infrastructure work is needed to better handle
register class constraints.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197514 91177308-0d34-0410-b5e6-96231b3b80d8