Re-enable "[MachineCopyPropagation] Extend pass to do COPY source forwarding"

Issues identified by buildbots addressed since original review:
- Fixed ARMLoadStoreOptimizer bug exposed by this change in r311907.
- The pass no longer forwards COPYs to physical register uses, since
  doing so can break code that implicitly relies on the physical
  register number of the use.
- The pass no longer forwards COPYs to undef uses, since doing so
  can break the machine verifier by creating LiveRanges that don't
  end on a use (since the undef operand is not considered a use).

  [MachineCopyPropagation] Extend pass to do COPY source forwarding

  This change extends MachineCopyPropagation to do COPY source forwarding.

  This change also extends the MachineCopyPropagation pass to be able to
  be run during register allocation, after physical registers have been
  assigned, but before the virtual registers have been re-written, which
  allows it to remove virtual register COPY LiveIntervals that become dead
  through the forwarding of all of their uses.

llvm-svn: 312154