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

Re-enable commit r323991 now that r325931 has been committed to make
MachineOperand::isRenamable() check more conservative w.r.t. code
changes and opt-in on a per-target basis.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@326208 91177308-0d34-0410-b5e6-96231b3b80d8
123 files changed