Reapply "[MachineCopyPropagation] Reimplement CopyTracker in terms of register units"
Recommits r342942, which was reverted in r343189, with a fix for an
issue where we would propagate unsafely if we defined only the upper
part of a register.
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.