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An identity COPY like this: %AL = COPY %AL, %EAX<imp-def> has no semantic effect, but encodes liveness information: Further users of %EAX only depend on this instruction even though it does not define the full register. Replace the COPY with a KILL instruction in those cases to maintain this liveness information. (This reverts a small part of r238588 but this time adds a comment explaining why a KILL instruction is useful). git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@274952 91177308-0d34-0410-b5e6-96231b3b80d8