[MachineCopyPropagation] Reimplement CopyTracker in terms of register units

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: 342942