Revert r342942 "[MachineCopyPropagation] Reimplement CopyTracker in terms of register units"

It seems to have broken several targets, see comments on the llvm-commits thread.

> Change the copy tracker to keep a single map of register units instead
> of 3 maps of registers. This gives a very significant compile time
> performance improvement to the pass. I measured a 30-40% decrease in
> time spent in MCP on x86 and AArch64 and much more significant
> improvements on out of tree targets with more registers.
>
> Differential Revision: https://reviews.llvm.org/D52374

llvm-svn: 343189