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.

Original message:

  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.

llvm-svn: 344942